Can prisoners testify in court? If so, are there any limitations on what they can and can't say?

I’ve seen on various cop shows that prisoners can and occasionally do testify in court. What I’m curious about is how accurate this is. I would imagine that, in some cases, prisoners have vital information that could be invaluable in court. So, can prisoners testify? Is there any limit as to what they can say in court? If there are limitations, do you know what they are?