well Radar lock is (on a basic level) for use with Aim-7 constant-lock, and Aim-120s (and 54s though they are going away) which need a shorter lock on. Aim-9s are heater (IR) missiles and do not require that type of Radar lock as they are not radar supported.
the Aim 7 is a semi-active radar guided AAM. The Pilot needs to keep the target locked up on the scope (MFD) multi function display (different names for different airframes but it is the same thing). Aim 7s are for shorter ranges than the AIM-120s (which are for BVR-Beyond visual range shots). Air Defense Versions of the F-16 can carry these and they are useful because if you shot them and then need to 'snip it' i.e. stop supporting it you can and it will stop tracking (maybe you took a shot at something you shouldn't have)
Otherwise the Aim-7 must be supported until it hits the target. i.e. the pilot must keep his radar on the target until F-pole, or the point that the missile will impact. So the pilot cannot get out of dodge until he sees the fireball.
The AIM120 does not need this constant support it is an 'active' radar missile and tracks on it's own. Though if you take an off boresight or shot way to the side then it will be a low percentage probability of kill. Otherwise the Aim-120 is much more leathal. And the pilot can get of dodge much quicker
Does that help???
Ciao and Hook 'em Horns,
Io voglio fica ogni giorni da mia bella moglie!