This refers to the way many apps and web platforms attempt to wall in their users, to keep them inside their own app or web platform and discourage them from going outside it. This is an affect of the advertising business model, where web platforms and apps are selling your time and attention for profit, therefore their main goal is to hold your attention and keep you on their platform as long as possible.
This term has especially been used in the case of Instagram, which does not allow external links at all on their platform. Once they suck you in, they just want to keep you there, and never let you leave.