it says the theme revolves around "portal pass", what even is that..

i think its like the game portal. you have to make a portal and teleport through it or  teleport things through it to progress in the level

I thought that at first, but I think you can do anything you want in your level. You just have to enter a portal (or something similar) at the end of the level, to "pass" the player to the next level, which will be made by someone else.

nope i think like they are going to make a loop of games , like every game must have a portal at last and if player enter to that portal then he will be redirect to another game made by other devs as tripo also mentioned it like we just have to made the last portal system and linking games will be done by the tripo .