Adjacency bonuses аre а new mechanic widely used in Civilization VI to simulate the beneficial or non-beneficial effect terrain and game objects have on each other. It plays into the heavy emphasis this game gives to terrain use and the planning of the new 'urban sprawl' in the form of districts and wonders being built out in the open terrain alongside tile improvements.
What are Adjacency bonuses?
Adjacency bonuses manifest in different ways for different gameplay elements; however, most of the time they manifest in terms of gameplay stats which are added to the element according to its surroundings. For example, 1 point of 
Science yield will be added to each Campus district for each Mountain tile near it.
Adjacency bonuses work for each concerned game element by looking up all six surrounding tiles each turn, and then calculating and conferring bonuses (or penalties) according to what is in these tiles. The bonuses, or penalties, are always in relation to the element itself: it may happen that the same tile with the same stuff on it will affect in 2 or more different ways nearby different elements! For example, a Mine improvement will give a nearby Industrial Zone +1 
Production bonus, but at the same time will lower the Appeal rating of all other nearby tiles by -1!