The internet isn’t exclusively a cesspool, but it contains a lot of them and they move around.
This is especially true of Twitter. The downside of being a hyper-popular network that theoretically anyone can be sign up for is that some nasty people register. A lot of those nasty people, probably most, are men, and a lot of those men target women for all sorts of abuse.
Some of that abuse is violent and explicit, but a lot of it is just annoying. For instance asking, if not demanding, that women take time to explain something, especially concepts or questions involved with feminism.
Elizabeth Simins, an artist who, amongst many other things, is an incredibly active Twitter user, has developed a tool to confront this: The Womansplainer. For various price levels, Simins offers to briefly search for answers to or have a brief public conversation with users about one of several questions about feminism, rape culture, and domestic violence.
The point of the joke, as Simins has explicitly stated, is to highlight that it’s not okay for men on the internet to demand that women use their time to explain things to men for free. Naturally, it’s attracted many angry voices decrying it or offering unsolicited advice on how it could supposedly be better.
Really, though, it’s a very good way of shining a flashlight on a type of abuse that’s all too often ignored or abetted by those who don’t suffer it.