A "front end design" is a high-level design of a product; more usually of a system. Depending on the brief, it may show the layout of a processing plant, the key operating mechanisms of a machine, or a packaging layout of a consumer product.
While it may prove such issues as feasibility, scale, dimension, concept, cost etc, it doesn't get involved with the 'nuts and bolts' of the product, any in-depth calculations (other than to prove the basic premise), or detail of individual components/subsystems.
So you might have have a front end design of an overall product, then perhaps system schemes, then assembly drawings, then detailed component drawings.
It must be a British term!