What does the term "uncomment" mean?

It is pretty obvious that I know nothing of code. On this forum, and sometimes in installation instructions, there is reference to “uncommenting” or “commenting” a line of code. For example part of the Ad widget installation instructions say:

*Note* – If you want to allow PHP code within the Widget – uncomment line 30.

If you want the widget to be only usable to supporters, uncomment line 31.”

My question is, what does it mean to “uncomment” something? I can see that it is very basic but I don’t know what it means. A short explanation will educate me. I tried looking in the manuals and in the codex but apparently this is so basic that everyone is just expected to know it. I didn’t find the answer. Ignorance is curable.