You will spend your time working in a least a few different applications and you will need to be able to switch between them quickly. The Mac has a built in keyboard shortcut Command + Tab. If you press it, you will see something like this:
This is the application switcher, which shows you all applications that are running. When you press it, press both Command and Tab, but let go of Tab as soon as you press it whil continuing to hold down Command. The switcher will continue to display until you let go of Command. Once you let go of Command, the highlighted application will come up.
If you press and release Command + Tab repeatedly, you will notice that you will jump back and forth between the current application and the previous application. It goes very fast, sometimes quickly enough that you don't even see the switcher. Play around with this for a few minutes until you get the hang of it. Switching between two applications frequently is something that you do a lot, such as switching between a text editor and a browser, for example.
Also notice if you press Command + Tab and continue to hold Command, each time you press tab, the switcher will move to the next application. Once you let go of Command, that application will come up.