It's something like: that's right. or you're right. or i agree with you. but sometimes, it can be used to indicate sarcasm as well. eg. suppose you say, i'm looking very beautiful today. don't you think so? then the answer may be... tei ta. but depending upon the tone the person is speaking, it may mean "yeah, you're really looking beautiful" or "are you kidding?" (but the second one is not a serious criticism.... just a funny expression. and it's usually accompanied by a smile or a laugh maybe)