Sortition means you dont stand to get elected, instead you get randomly selected to serve, its how jury service works. That said I'd prefer 2/3rds sortition 1/3 appointed cross-bench experts, they are the only good thing about the current lords and it would be a shame to lose them.

That's pretty much what it is - you can think of it like political jury service. It would be a cross-section of the population vetting laws with an eye on how they impact the lives of ordinary people. So you might get an NHS doctor pitching in on medical reforms but you also get a lot of people who use the NHS and are incentivised to improve it because it is life or death to them, rather than a politician who might be more interested in seeing what can be flogged off to their mates in return for a cushy non-executive position.

We largely only get a say in the laws during elections when we have to decide who we trust yo make the changes we want. With sortition in the upper house we get instant and fine-grained oversight of each individual law.