One of the ways information is blocked from retrieval from your long-term memory.
In retroactive interference new information blocks old information. (retroactive = acting backwards in time) For example, you can waste hours memorizing pi, only to lose it because you try to memorize a phone number afterwards--your numbers start to get mixed up, and soon only the newer stuff can be accurately retrieved. Retroactive interference is worst when the later item is learned shortly after the earlier item.
Compare to Proactive interference.