In the Japanese exclamation:
待ってろよ！
What function does ろ have and how should it be translated to English? I've searched everywhere, but I can't seem to find a decent explanation. The only thing that came close was a website telling me that the て-form could sometimes be replaced by a ろ, but that's not what's happening here..
This is a shortening of 待っていろ which is imperative for 待っている (いろ being the imperative of いる)
「[待]{ま}ってろよ」is a colloquial contraction of 「待っていろよ」. It is the "tough guy" speech, so to speak.
There is a difference in meaning between 「待て/待って」and「待って（い）ろ」. The former is the simple "Wait (a second)." while the latter means "You wait (there for a period of time) for me (or someone/something to arrive)." The latter is actually ordering one to "stay" there. I am sure that you can see two verbs in 「待っていろ」, which is 「待つ = "wait"」and「いる = "stay"」.
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.
吉川武時(1979)「『していて下さい』の意味――『待って下さい』と『待っていて下さい』の使い分け――」,『日本語学校論集』,6, pp.62-91.
(Actually his analysis is different from mine.)
