Adjacency refers to two elements that are next to each other in the surface string without any intervening material. This notion is relevant to Case assignment.
(i) John wrote a letter yesterday.
(ii) *John wrote yesterday a letter.
The NP a letter must be adjacent to its Case assigner wrote, as in (i). If any material intervenes, as in (ii), the adjacency requirement on case assignment is not met, hence the NP cannot be assigned case so that it violates the case filter, and the sentence is ungrammatical.
One distinguishes string adjacency from structure adjacency depending on whether the relation 'next to each other' applies to a string or to structure.
Chomsky, Noam A. 1981. Lectures on Government and Binding. Dordrecht:Foris.