Trust Level 3 means you are a "Regular Visitor" to the forum. Level 2 just means you're a "Member"
You apparently have to be a "Regular" to access The Lounge, it's a concept like a "Frequent Flyer Club" in Discourse, the software that drives the forum.
Level 3 can be granted and removed by the software, I'm surprised it's independent of admin status though.
Details of Discourse trust levels can be found here:
Given the daily traffic here, those default settings are kinda off-base. I generally don't reply to topics unless I think I'm adding something, and I'm not fixated on giving and receiving likes as the younger crowd seems to be... actually I'm not sure this forum even needs a frequent flyer club. It might be good for something like issuing coupon codes to high contributors. But do we really need elitism?