"I happened to be there, so I was able to help them." works better, but "could" has essentially the same meaning. "Could" is past tense of "can", however you will also see it used in phrases like, "If I could do that, I would." I'm not a grammar expert, but I believe in that latter case it is being used as an auxiliary verb to indicate a conditional mood in English. English verbs are not inflected much, so we have all these confusing helper verbs. Also, the verb "can" is confusing because it often has to be changed into "be able to" in different tenses because, for example, "can" has no future tense. You cannot say "I will can". I digressed a little. The simple answer to your question is that in this case "could" is the simple past of "can".