People will give you all sorts of misinformed explanations. Matt Levine, the Bloomberg columnist, is a respected guy who understands the industry. Here's his take, a trustworthy take, on it: https://www.bloomberg.com/opinion/articles/2018-10-16/carl-i...
(skip forward to the part where it says "Robin Hood", it's just part of his column that day)
tl;dr You're not getting screwed by Robin Hood (as a matter of fact, they're good for you), but Robin Hood is enabling you to make trades you shouldn't be making in the first place because you're probably unsophisticated.
I agree that I’m unsophisticated but not that I shouldn’t be trading. Most of my investments are 401k and passive index funds. But I keep a small amount of active trading capital to learn by doing. Sometimes I win, sometimes I lose. But it’s a hobby that I enjoy. And I’m becoming more sophisticated over time.
Ok, so after reading the Bloomberg article, your 401k is being negatively impacted by order payment flow. You as an individual retail investor is probably ok but your 401k is losing in aggregate.
(skip forward to the part where it says "Robin Hood", it's just part of his column that day)
tl;dr You're not getting screwed by Robin Hood (as a matter of fact, they're good for you), but Robin Hood is enabling you to make trades you shouldn't be making in the first place because you're probably unsophisticated.