As other people said, てろ is short for ていろ.
I think there are mainly too ways to use it.
It means "keep on doing/continue to do". It's used when the listener is already doing something, and you ask him to continue. I feel this is somewhat used rhetorically. It means you don't really want to stop him.
It implies something might happen during (and might interrupt) the process, or you just try and see and may interrupt the process. I think 待っていろ generally carries this sense.
(I have a meeting to attend, and will later post more examples when I come back.)
I just found a paper about the difference between しろ and していろ. I'm not sure if the question is really asking about it, so I'm not going to add additional explanations.
For those who want to learn more, click the following link.
(Actually his analysis is different from mine.)