The word mansplaining comes from the words man and explaining. In 2010 is was named as one of the words of the year by the New York Times. It is not a very positive word, it is condescending.
It is typically used when addressing a woman, but this is not always the case. It is a way of explaining something which was already clear, of explaining it in a manner that seems patronizing. The explanation is not necessary since all the present parties already know the meaning of what is explained.