This is a very fun way to learn and get better. It's still in a very active development stage, so things go up-and-down regularly (which sucks when you want to get your fix), but it's mostly been awesome while I've used it the last few months or so.
If you do try it out, one of the best parts is adding your own "kata" — basically the games or tests. You will learn a lot by writing them (the community is very active in helping users improve description text and test cases). And there's nothing like seeing someone come in and absolutely blast your best effort out of the water with something even more amazing.
It's a slight difference, but it's more about getting 500 people (unique or recurring) to give you (an average of) $30 per month.
This could be automated recurring revenue (like SaaS), or building products that are one-time or intermittent purchases with many customers. Or, you can try to get fewer customers at a higher price, or more customers at a lower price for some things (but that's almost always harder to make it work).
It's also about building products that aren't full-time jobs in and of themselves, so you can have several products over time, that cover a range of concepts and audiences. This has the added benefit of providing you with diversified income, so you aren't SOL if a single product starts to lose customers.
Ebooks are a common initial product, because they have a little less initial risk and are a little easier to start as a side project than a full-fledged SaaS product.
If you do try it out, one of the best parts is adding your own "kata" — basically the games or tests. You will learn a lot by writing them (the community is very active in helping users improve description text and test cases). And there's nothing like seeing someone come in and absolutely blast your best effort out of the water with something even more amazing.