Emacs uses the terms point, mark, and region to provide more precision about the selected text and position of the cursor. By understanding these terms, it'll help you understand and use other operations and functions.
The point is the place in a buffer where editing (i.e. insertion) is currently taking place, and is usually indicated by a cursor.
The mark is a marker placed anywhere in the buffer using commands like
set-mark-command (C-SPC) or
exchange-point-and-mark (C-x C-x).
The region is the area between point and mark, and many commands operate on the region to e.g. delete, spell check, indent, or compile it.
When you click your mouse on a location in a buffer, you're seeing the point. When you select text, you're setting the region (the selected text) and the mark (at the beginning of your selection).