Some keys are modifier keys: they change the meaning of the other key you press. A clear example of this is how you use the shift (⇧) key to type the lower and upper case letter A:
Both “a” and “A” are printable characters — good for typing words. But other modifiers can indicate that you’re not trying to type the letter “a” at all, but using it as a button to have some effect. You are sending a control signal.
For example, if you are pressing the control key when you hit the A, it might mean “select All the things”. The specific meaning will vary depending on what program is currently running, and what it’s listening for. To help you remember, programs tend to use the same conventions for what the key combinations mean.
You can only do combinations with modifier keys. Pressing more than one ordinary key at the same time (for example, A and B) generally does not work.