In the context of a new bike spec list or marketing copy, it's most commonly understood to mean a wheelset and tires that are both tubeless-compatible, plus the rims already have tubeless tape. It's ready for tubeless in the sense that all you have to do is unseat one bead, remove the tube, add a tubeless valve, add sealant, re-seat the one bead, slosh the sealant around, and go. Manufacturers have gotten pretty good in recent years at making this actually work as planned, i.e. the factory tape job is good. That's not an absolute given though; it's always a good idea to look at it and make sure the tape application and coverage are something you want to commit to.
What's a little inconsistent with the term still in my experience is whether the bike comes with valves in its goody bag. Some do, particularly if it's got a named prefab wheelset, but a lot don't, so if you for example want to make sure you're ordering everything you need to tubeless it right away, get valves too unless it says it includes them.
If the term is being used for a wheelset, I would take that to mean it comes with tubeless tape. For a tire, it just means it's a tubeless compatible tire. (If you look around and read old things you might see some reference to "tubeless ready" meaning a tire with conventional non-sealing sidewalls, as opposed to UST tires with their extra sealing layer for use without sealant, but nobody really cares about distinction anymore.)