A tricategory is a particular algebraic notion of weak 3-category. The idea is that a tricategory is a category weakly enriched over Bicat: the hom-objects of a tricategory are bicategories, and the associativity and unity laws of enriched categories hold only up to coherent equivalence.
One way to state the coherence theorem for tricategories is that every tricategory is equivalent to a Gray-category, which is a sort of semi-strict 3-category (everything is strict except for the interchange law).
The monoidal product is given by tensor product over .
The tricategory statement follows from theorem 21 of
This, and that the monoidal bicategory is even symmetric monoidal is given by the main theorem in
The original source is
This was refined in the thesis
which is probably the best current starting point to read about tricategories and from where to take pointers to the original work by Gordon-Power-Street.