Resolution means the ability of the radar to pick out targets; a higher resolution means the radar is better at this. The radar's resolution will be better if it can distinguish between targets that are close together. Firstly, if the width of the beam is too wide (think of it like a fan) then the beam may envelop two targets that are close together, giving one return rather than two. So a narrower beamwidth makes for a better resolution. Secondly, if the radar transmits for a long time (long pulse length) then that pulse may envelop two nearby targets, giving two returns rather than one. Therefore, a shorter pulse length gives a better resolution.