Those small, flea-like insects are almost certainly aphids.
Many ants form relationships with aphids and care for them in much the same way humans care for domesticated livestock like cows and chickens. Aphids use their sharp mouthparts to pierce plants and feed on the sap inside. The pressure of the sap is strong enough that it actually pushes its way through the whole digestive tract of the aphid, forming a bead of sap at their bottom end. This sugary liquid is called honeydew, and ants that form relationships with aphids feed on it. Just like cows and chickens give us milk and eggs to eat, aphids give ants honeydew to eat.
The relationship is mutually beneficial. Ants get food in the form of honeydew, and aphids get protection and increased mobility. If sap starts running dry in one area of the plant, the ants will move them to another area. If it starts to rain, the ants will move the aphids to the undersides of leaves and other sheltered parts of the plant so they won't be washed away.
It's really a fascinating example of two different insects cooperating. Ants are pretty unique in that they form relationships with many different types of organisms. Various species form relationships with caterpillars, acacia trees, and fungi that are mutually beneficial.