Inter-arch (intermaxillary) elastics are an additional way of moving a particular group of teeth. They get their name from the fact that they attach to the upper and lower braces at the same time. They can be placed in many different ways, and the way they are placed determines the effect they have. If an elastic runs between the lower back braces and the upper front braces, it is probably being used to correct the bite in the front-to-back direction. If it is hooked onto opposing upper and lower teeth, the force from the elastic acts vertically and draws the teeth down – to correct an open bite, for example.
This answer is for general information. Your own case needs an examination by an orthodontist.