How come that someone understands an utterance only after a few moments? I have seen and experienced the following phenomena. It often happen in a work environment but can happen anytime and anywhere:

1) Person A is doing something that requires full concentration (like reading or thinking deeply).

2) Person B suddenly ask/say something to Person A

3) Person A replied with "What do you say?" or "Pardon please" (or something similar, as if he/she doesn't hear what person B said) after some brief pause/delay.

4) Person B didn't answer back (optional)

5) Person A answers/responses (before Person B repeats what he/she said) to Person B (as if he/she heard it fully)

Is there a psychological term that describes/explain it? How it is happening? What is its effect with the brain of Person A if there is?

P.S.
I don't know what is the right tag for it.
I think what you describe can be explained by echoic memory and working memory.


  Echoic memory is one of the sensory memory registers; a component of
  sensory memory (SM) that is specific to retaining auditory
  information. The sensory memory for sounds that people have just
  perceived is the form of echoic memory. ... This particular sensory store is capable of storing large amounts of
  auditory information that is only retained for a short period of time
  (3–4 seconds). This echoic sound resonates in the mind and is replayed
  for this brief amount of time shortly after the presentation of
  auditory stimuli.


. 


  Working memory, a core executive function, is a cognitive memory
  buffer with a limited capacity that is responsible for the transient
  holding, processing, and manipulation of information. Working
  memory is an important process for reasoning and the guidance of
  decision making and behavior.


.

Person A is fully concentrated and his working memory capacity is full, so he does not pay attention to what the other person is saying. But when person A asks "What do you say?", working memory resources are released and attention is diverted to the echoic memory which replays what person B said.