Top 30 is what they call the post Combine/pre draft extended interviews/individual workouts/physical by team doctors. Each team can bring in up to 30 non-local prospects. They are most meaningful because they are spending the most time and resources with these players for a reason.
It doesn’t mean top 30 prospects on the big board or anything. They can bring in whoever at any position and they won’t all be top prospects. Sometimes they might bring in a teammate of someone they are really targeting in the 1st for example to get extra intel even if they aren’t interested in that player. Olave, Wilson and Williams are very tight knit - if they bring in all 3 I won’t be surprised if they are really just honed in on one of them.