A table function (some times also called "table valued function") is a stored function that returns a result set that can be used anywhere where a table can be used

select *
from some_function(42,'some_argument') f
  join orders o on o.order_id = f.order_id
where f.some_column > 100;

some_function is a function written in the procedural (SQL based) language available in the DBMS (PL/SQL, PL/pgSQL, T-SQL, ...)