The Grothendieck construction gives a way of gluing together the constituent categories $F(x)$ of an indexed category$F \colon X^{op} \to \mathsf{Cat}$ to get a category $\int F$ which admits an obvious fibration over the base $\int F \to X$. This gives an equivalence between the 2-category of indexed categories and the 2-category of fibrations.

Two ways monoidal structures can join this story is on the total category $\int F$, or on the fibres $F(x)$. The 2-equivalence then lifts to two monoidal variants, one where the fibres are equipped with a monoidal structure, and one where the total category is equipped with a monoidal structure. Under certain conditions on the base category, these two settings are equivalent to each other as well. That is to say, under the right conditions, one can glue together the monoidal structures on the fibres to get a monoidal structure on the total category.