The activities overview has three functions: launch applications, see your open windows, and quick launch a Google or Wikipedia search.
To access the activities overview, simply push your mouse cursor into the top left of your screen. Or use the "super" key (the "Windows" or "Command" key).
On the left side of the activities overview is your Favourites Bar. This is like the Dock in Mac OS, or Shortcuts in Windows. This bar is for apps that you use often and want to be able to launch quickly.
There are two views of the activities overview; you can look at the open windows (they appear in the middle of your screen, floating above your desktop) or you can see available applications. Try switching over to the applications by clicking the Applications button.
You might recognize some applications, like Firefox, and others you'll be able to infer what they do (like Dictionary, Files, Backup, and so on). Others, you'll learn about soon.
In the top right corner of the screen is a search field. If you know the application you want to launch, you can just start typing (no need to click in the search field) the name of the app and the closest matching apps will be suggested.
On your computer, try dragging an application over to your favourites bar.
Now try rearranging your favourites bar. It takes a little bit of technique (it's all in the wrist), but if you click AND HOLD the mouse button on an icon and then drag it up or down along the bar, you'll find that you can indeed rearrange the order.
Try removing an application from the favourites bar. This one's a little trickier, but try clicking AND HOLDING the mouse button on an icon in the favourites bar until a pop-up menu appears. One of the choices should be Remove from Favourites
Try finding and launching the text editor ("Text Edit" on Mac, "Notepad" on Windows). See if you can figure out which application it is just from the icon.