This is much like an echo or repeater type shape creation. Apply to a Shape on a Shape Layer, interpolates a set number "steps" between it and another Shape in a different position and/or configuration.

Example, a straight Line "Blend"-ed with 5 steps to a wavy Line at a different place in space will produce 6 Lines (if the first is counted as a step) with the first being a straight line and each line placed in space between it and the wavy Line. Keyframe it for super cool animations!

reference Blend Tool in Illustrator: https://helpx.adobe.com/illustrator/using/blending-objects.html