Mission Control Spaces are essentially different iterations of your desktop that can all display different apps and windows. If you find your current desktop is getting a little crowded, but you don't want to close the apps and windows you've got open, you can create a brand new Space to work with. Some people will even create Spaces for different tasks.
F3 is the Mission Control key on your Macbook keyboard.
You can also swipe up on the trackpad with two fingers on a laptop.
This will temporarily sort all of the windows you have open so you can see them all at once. From here you can create another desktop, or 'Space', to work with.
To create a new Space, open Mission Control and click on the + (add button).
This will create another desktop at the top of the screen for you to switch to.
To move apps to another Desktop, you can drag and drop their open windows from Mission Control right into whichever Space you would like them to be in.