The basic flam is a foundational pattern that you'll need to learn in order to play more advanced rudiments. You won't be able to play the flam accent, flam tap, flamacue, single flammed mill, flam paradiddle, flam paradiddle-diddle, or several other rudiments without first learning the basic flam.
A flam stroke consists of two notes being played as one. The first note is smaller to indicate that it is to be played at a lower volume, while the second is to be played at a normal volume. The trick is to play the notes slightly off-set, so they have a fatter sound. Watch the training video to see exactly how this is done.