#javob

It includes non-index column values to the actual index (b-tree).

Let's say you have a table named "users" with id, name and a bunch of other columns.
If you include name column in the index on id field:
CREATE INDEX
idx_user_id_include_name
ON users(id)
INCLUDE name;

Each time you query that uses this index and only asks for the columns on the index:

SELECT id, name 
FROM users
WHERE id = 1;

PostgreSQL doesn't necessarily go to the actual table after finding the required row, since it can find both id and name field on the index. This is called index-only scan.

If you query some other columns (let's say age), PostgreSQL first finds the required row id from the index and go to that row on the table to get age value, since it's not presented on the index. Searching on the index and fetching from the actual table. This is called index scan.