What is an "ACCESS METHOD"? In the context of the CREATE ACCESS METHOD DDL in PostgreSQL,

CREATE ACCESS METHOD name
    TYPE access_method_type
    HANDLER handler_function


What is a a high level description of what an "access method" is and what they are used for?
Using CREATE ACCESS METHOD in postgres allows you to define a function to handle (or 'access') a type of index.

It exists to allow you to define new ways for postgres to understand a mapping from the data key values to the actual TID that identifies the related value.

Without an understanding of how relational databases work, it's probably not something you'll ever directly deal with.