Event comments are simply notes to help you remember how events work and what they do. They are very important to help you remember how things work in large projects! Comments have a yellow background and appear above the event they are describing.
To add a comment, right-click on an event or an empty space in the event sheet and select Add comment. Alternatively use the Events tab on the ribbon or press the Q keyboard shortcut. Comments can be edited by double clicking on them. You can make a comment with a line break by holding shift and pressing Enter.
If you use Construct 2 a lot, you will find comments essential to help yourself organise and understand large projects. Coming back to a project after a few months with no comments at all can be very difficult, so don't underestimate the importance of comments.
Comments do not affect how anything works at all. They are solely for your information. Nothing typed in to comments is exported to the game whatsoever.