I have experienced this phenomenon several times and checked with other people as well.
It goes like this: you hear something, but it's just a sound with no meaning. Some seconds later, you consciously remember the sound and make sense out of it (like a word or a phrase that somebody tells).
Although both processes (remembrance and understanding) are fairly common, I find it weird that understanding is not automatically triggered when processing a sound and it can be demanded on purpose
Is there any name for this phenomenon? (Would appreciate some detail on it too.)