The term front end usually refers to a small computer attached to a large one (usually a mainframe), which is used to offload input/output work from the larger machine. (Hence the alternative terms I/O processor and peripheral processor.)
There are several advantages to offloading I/O activity to a separate computer. First, the obvious one is the ability to bring additional processing power to bear. Second, large mainframes are typically optimized for doing large-scale computations, not doing I/O. Also, the overhead involved in processing an interrupt from a device (e.g. saving register contents) can be significant, so avoiding bothering the main CPU with this (often larger in a mainframe than in a smaller machine) can produce significant efficiencies.
The front end machine can communicate with the main CPU in a number of ways. It may have direct access to the main memory used by the main CPU, a technique used in the PPUs of the CDC 6600, the DL10-PDP-11 combination of the PDP-10 family, etc.