Sometimes it could mean that there is a movement coming in the opposite direction. It could also mean that something has goe passed that signal and is still within the same block. Either way the signal could be telling you that there is a train in that block. But many times signals will remain Red (Stop Signal) if there are no trains anywhere near you if the dispatcher doesn't have any trains coming for a while.
Hope this answers your question. I am assuming that you are just using the signals as a way to determine if anythingwill be coming or not.