Emacs has two kinds of modes major and minor. A major mode is like c-mode, org-mode or text-mode. It can probably me something other than text editing. But in general a major mode is what you use to edit a a file of a given format. For example python-mode is a mode you can use to edit python files, and it has functionality that makes that easier and better. You can only have one major mode active at a time. A minor mode on the other hand add some kind of functionality. Company is a minor mode, yasnippet is a minormode, projectile is a minor mode. You can have as many minor modes active as you want.