In general, the term "mansplaining" isn't merely referring to a man explaining something. Being a man explaining something and mansplaining are different experiences (although there is obvious overlap). Mansplaining largely involves a man explaining something to a woman that she already knows, under the assumption that she needs it explained.
It's a gendered experience that usually combines an extra dose of male confidence, and embedded sexist notions of how much expertise women have in their fields. Due to the nature of mansplaining, a lot of men don't realize when they're doing it.
In fact, the writer Kim Goodwin has been asked about the definition of mansplaining by so many male peers she decided to make a nifty chart.