A shop sells each tee in every size and every colour. You need every pair:

SELECT s.size, k.colour
FROM sizes s
CROSS JOIN colours k;

CROSS JOIN has no ON. It pairs every row of the first table with every row of the second. The result is the Cartesian product of the two tables: rows of A × rows of B rows.

Older SQL writes a cross join with a comma:

SELECT s.size, k.colour FROM sizes s, colours k;

A join is a product plus a filter

An inner join returns the pairs of the cross join where the condition is true. These two queries return the same rows:

SELECT … FROM orders o JOIN customers c ON o.customer_id = c.id;
SELECT … FROM orders o, customers c WHERE o.customer_id = c.id;

The database does not build the full product to run a join. The result is the same as if it did.