ls
List all the files and subdirectories in the current directory.
cd Desktop
From the current directory, go to the subdirectory named "Desktop". This is just an example and assumes there is a subdirectory called "Desktop". Note that if you type the first few letters of the subdirectory name and then press tab, the name will get autocompleted.
cd ..
From the current directory, go one directory up in the directory structure.
python test.py
Run the python file "test.py". This assumes that the python file "test.py" is present in the current directory.
./test.sh
Run the executable file "test.sh". This assumes that the file "test.sh" is present in the current directory.