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.