To marinade something generally means to let it soak in something else and absorb it. This idea is usually applied to food. An example would be, marinating a piece of chicken in an orange marinade.
As a poetry topic idea though, a more broad meaning of the concept can be looked at. If the idea of letting one thing soak in another to absorb it is seen broadly and metaphorically, and not just applied to food and in a literal sense, it can be used in poetry.
Some examples of this might include poems about someone sitting and listening to music, someone swimming in a lake or someone at a happy occasion. The idea would be that the person in the poem would, in a way, be absorbing something emotional from their surroundings.
Here is an example poem that uses the idea of marinating:
Walking by each gate,
the little dogs jumped and barked.
As she thought,
the cute sounds of “Please pick me!”,
soaked into her.