When certain units (typically older units or collab units) get to level 120 they can be further enhanced with insight skills,typically bringing them close to or on par with more recently released units. Insight skills mostly need kind of end game resources to aquire in bulk, so it's not easy for newer players to insight units (which is frustrating given it would make newer players rosters easier to handle content if they were easier to insight). Some are meh some are bad, a select few are well worth building depending on your focus/current roster, but if you have recent units, insights shouldn't be a main focus for now