a.b is the composition of the relations a and b. For each tuple in a of the form (a0, a1, ..., an, X), and each tuple in b of the form (X, b1, ..., bn), the new relation a.b contains a tuple (a0, a1, ..., an, b1, ... bn). Only the last atom in tuples of a are matched with the first atom in tuples of b. Note that the matched atom is removed from the resulting tuple.
Due to some clever definitions of Semantics, it can also be used to access fields of a signature. See levels of understanding for examples of different ways to interpret the dot.