Thanks Lego!
The "Because you read" feature is driven by the Cirrus MoreLike api. Basically when you refresh the feed we send a query to MoreLike, and put the received results into the feed. We do not build or maintain any user specific browsing history or interest profiles on the backend. On the client device we keep a browsing history (which you can delete via the "Clear" button on the History tab). This means our recommendations are a bit less "personal" than other similar systems, but privacy friendly and built with compentents already available in the MediaWiki/Wikipedia stack.
After we have more info about the Explore feed and how people are using it (via those opt-in analytics and qualitative feedback) we'll be starting an on-wiki conversation for future directions that maintain our privacy commitments but deliver "good stuff" to read.