I don't even know what it is.

I do know that it affects file size.

I tested different bitrates with sample video of me fixing a laptop.

129,000 kbps = good
10,000 kbps = good
4,000  kbps = good
2,000 kbps = just starting to pixelize
300 kbps = full pixelized

Note: 10,000,000 in Vegas = 10,000 kbps in Properties

Does this depend on the machine viewing the video?

What situations require different bitrates? When would you ever choose 300? Does it depend on what is in the video? Static interview vs. panning movement?

I just looked at the bitrate of my source video file from my phone. It has 16,000 kbps. So, my rendered video probably should not be higher than that!    But, it looks like I can go down to 4,000 and potentially make it 25% of the size.