I've seen mansplaining used two different ways. One is when you are discussing an issue that is specific to women, or that has a strongly female slant on it, and a man pops up to tell you how you are totally wrong about what the issue is, and your reaction to is is also erroneous.
So, for example, a group of mothers discussing breastfeeding, and perhaps the feelings they get when feeding in public. And a man popping up pointing out that they're wrong to feel that way, and they know of A Woman who didn't feel that way, ergo your experience is much less valid than his.
I've also seen it go beyond that to where a woman has specific expertise in a certain area, but a man will pop up to explain it all again, completely unnecessarily, and often getting it wrong. In this situation, it is that the man simply believes he knows best on account of possessing some testicles.
So, for example, a woman with an engineering degree working on some code, and a man with no expertise popping up to point out that the problem with the computer is almost certainly a software conflict, because he's had that happen once.
It's particularly annoying when a second man pops up to back up the first, or to hang on the new 'experts' every word.