A focusing cup is a negatively charged, shallow depression on the surface of the cathode of an x-ray tube, which concentrates the electron beam towards the focal spot of the anode. It is typically composed of nickel.
The negative charge of the focusing cup helps to accelerate the electrons towards the anode.
The shape of the focusing cup helps to converge the electrons onto the focal spot. Otherwise, electrostatic repulsion between the negatively charged electrons will cause them to spread out as they reach the anode, resulting in only some reaching the target.