Simple question about the IDE

I got my first Arduino board this afternoon. It took almost 5 minutes to get it up and running from my iMac - very impressive. I have been playing with it for the last hour.

My question is in regard to the IDE window which has 5 sections. The bottom one (green) shows a number which seems to vary randomly as I move from one window to another after editing. What is it?

The line number of the source file that the cursor is on. It does not change randomly. You just haven’t detected the pattern.

While you're typing, look at what happens to that number when you go up or down a line.