It means that on the conflicting traffics current track that it is moving from left to right in relation to your track. Generally speaking if a conflicting track is much slower than you I'll tell you that it is "slow left, right." If I think it is pertinent I'll tell you that it will pass behind; however, according to the letter of the law all I have to do is call the traffic to you, using "standard RT" and update you only if you request it or I deem that it still poses a threat to you, because you've not called visual or not done anything about it!