Some of the text below was taken from this page.
The EV3 Explorer is a separate program that has several functions:
You do NOT need to use EV3 Explorer to run your EV3 Basic programs on the EV3 - just press F5 or hit the Run button while in Small Basic. However you will want to use Explorer if:
Important note: The standard Lego EV3 software (EV3-G) demands an exclusive connection to the brick so if the Lego software is running you will not be able to run any EV3 Basic program from Small Basic or EV3 Explorer. Always remember to close the standard Lego EV3 software before attempting to run a program from Small Basic or EV3 Explorer.
Similarly, if Small Basic and EV3 Explorer are running at the same time then EV3 Basic will have an exclusive connection to the brick and it will not be possible to run EV3 Basic programs from Small Basic. This does NOT mean that it is always a bad idea to run both these program at the same time, however. If you are working in PC mode then clearly you don't need EV3 Explorer but if you are working in brick mode (as is preferable if you are using a wireless connection) then it's very sensible to have Small Basic and EV3 Explorer running at the same time. You can then easily save your program in Small Basic and immediately (compile and) run it in EV3 Explorer without having to close any program.
It is even possible to start programs in PC mode while EV3 Explorer is open. You can do that when you have multiple connection methods available in your setup. For example you can use EV3 Explorer with WiFi and the Small Basic program can use the USB connection. Nevertheless you cannot start two programs at the same time.
The EV3 Explorer program will be installed at the same time that the EV3 Basic extension is installed. EV3Explorer.exe will usually be placed inside this folder:
C:\Program Files (x86)\Microsoft\Small Basic\lib\EV3Explorer\
For easier access, you could make a shortcut to the program and put the shortcut on your desktop, for example, or in the task bar.
The main window is divided into two panels. The left one shows the file system of the EV3 and the right one shows your local PC file system. Both panels have navigation buttons at the top). It is possible to use the Ctrl and Shift keys in the usual way to select multiple files. You can also use Ctrl-A (select all) in the usual way. The software allows you to select files but not folders - if you click on a folder it will simply open. In order to delete a folder on the brick you must first enter it and delete all its contents - you will then be able to delete the folder.
In the left-hand panel above, note how a new folder called 'EV3-Basic' has been created inside the 'prjs' folder to hold the programs that have made in EV3 Basic. You need to put your program files in such a folder in order for the EV3 brick's software to find them.
Note that when you use the standard Lego EV3 software you make 'projects' that can each contain multiple 'programs'. It's important to realise that a project is just another name for a folder.
Note that there are folders called 'Images' and 'Sounds' on my brick. These folders contain the standard Lego sound and image files, ready to be used in my EV3 Basic programs. See the bottom of this page.
Before the brick can run your programs they have to be converted (compiled) from the Small Basic format to the RBF format that the brick understands. Before you compile the SB program to the brick, make sure the EV3Basic folder is open on the brick (left panel of EV3 Explorer). To compile a Small Basic program file (with extension 'SB' or 'SmallBasic') into the RBF format and then download it to the brick, select an SB or SmallBasic file on the PC side and click 'Compile' (or 'Compile and Run' to run it instantly). If you compile a program to the brick and a file with the same name already exists in the destination folder then that file will be replaced without asking you for confirmation.
Most programs will compile to the EV3 without problems as long as you do not use functions that are unsupported by the brick. If you try to compile a Small Basic program that uses commands that won't work on the brick such as commands involving the graphics or text windows, you will get an error. Such programs can be run directly from Small Basic (in 'PC mode') but won't work if you try to compile them into the brick to run them in 'brick mode'. There are also subtle differences for some language constructs which will also cause compilation to fail with an error. More details HERE.
Use the 'Download' button only when you want to transfer into the brick other file types that are compatible with the brick, such as sound files that your program will use. Remember, for Small Basic files you must use the 'Compile' or 'Compile and Run' buttons - DO NOT use the Download button with SB files. You will probably never need the 'Upload' button because your PC can make no use of RBF files.
You can use the textbox at the top of EV3 Explorer to easily rename your brick. The naming rules are the same as the rules for files (above) except that spaces are not allowed and the length may not exceed 12 characters.