That means if we take type IIA and Type IIB theory and compactify them both on a circle (one with a large radius and the other with a small radius) then switching the momentum and winding modes, and switching the distance scale, changes one theory into the other. The same is also true for the two heterotic theories.
This entails compactifying both Type IIA and Type IIB theories on a circle, one with a substantial radius and the other with a minimal radius, and swapping momentum and winding modes, along with distance scales. This interchange transforms one theory into the other. Likewise, this is also true for the heterotic theories.