Ask HN: Pure Math Problem

I'm reading A Course of Pure Mathematics, by G. H. (Godfrey Harold) Hardy – available free on Gutenberg. In the first chapter he presents the following problem (on rational numbers):

If λ, m, and n are positive rational numbers, and m > n, then λ(m^2 − n^2), 2λmn, and λ(m^2 + n^2) are positive rational numbers. Hence show how to determine any number of right-angled triangles the lengths of all of whose sides are rational.

I'm not asking the solution, but I'm not sure what he means by "any number of right-angled triangles"