Adjacency bonuses, when I choose my policies, theres ones like '+100 campus district adjacency bonuses'
I know this will sound thick, but am I supposed to place my campuses so that they are adjacent to the next city's campus. so if you have two cities close together, the one on the left I put his campus on the right and the right hand city I put his on the left so they are adjacent tiles?
or does it just mean the +1 and +2 that you see on the squares when you are choosing the location?
It is the +1 or +2 that you see when deciding where to plant a district. Adjacency bonuses vary by district type. For campuses, you get +1 science yield for each adjacent mountain tile, +1 science for every 2 adjacent rainforest tiles (and it is only for 2 rainforest tiles; you don't get +0.5 for being adjacent to 1 rainforest tile), etc. -- it's all in the Civilopedia. You do get +1 science for every 2 adjacent districts, which includes adjacent districts that were built in another city, but you would likely sacrifice other potential adjacency bonuses if you tried too hard to have adjacent campuses in neighboring cities.