Researchers wanted to determine if knowledge of a political party (measured on a continuous scale from 0-13) was dependent on different levels of mainstream and alternative media usage. Mainstream media use was measured in 3 levels (don't use, low use, high use) and alternative media use was measured in 2 levels (don't use, use).

The F test for the interaction between mainstream and alternative media use was F(2, 1907) = 6.639, p<0.001. How do I interpret what this means??