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.

This isn't intended to be my only product -- ebooks have a pretty vicious falloff in sales after launch. It's a chance to get all my marketing ducks in a row for when I do sell software.