Some examples of common windows.
A window is a box onscreen that holds content. This can be a web page, document, picture, group of pictures, whatever. Windows have some parts that are handy to know.
This can be dragged around to move the window. It contains the window controls, and may have a toolbar for getting things done and a title.
These three lights control the window. The red one closes the window. The yellow one moves the window to the Dock so you can come back to it later. The green one expands the window to take up the entire screen, or makes it smaller again.
These thin black lines define the edges of a window. When the cursor is on a window border, it changes to a set of arrows (⬌) to show that you can drag to change the size of the window.