What is the meaning of agents of pollination?

In short, that is a reference to bees and small birds.

Plants need to be pollinated so they can produce fruit/seeds. A plant contains a pistil and/or a stamen. Genetic material has to be carried from the stamens to the pistils. Sure, the wind can do some pollination or plants might pollinate themselves when they are bumped. But generally, it takes some creature to carry the pollen between plants. So that is where birds and bees come into the discussion. Bees, related insects (hornets, wasps, etc.), and small birds carry pollen between plants. This is how it must be done because a tree or other plant cannot simply get up and walk over to another plant of the same type and mate with it.