irLock is for cfgAmmo, it determines if a type of ammo will lock on or not when fired.
e.g. a free fall bomb does not lock on, a missile will lock on and follow the target.
canLock is for cfgWeapons, and determines if a particular weapon will lock onto a target.
canLock=0 no lock, canLock=1 lock in cadet mode only, canLock=2 lock always
You can use canLock=1 or canLock=2 and irLock=0, this means the weapon will lock on but the ammo wont home in on the target.
Or you can use canLock=1 or canLock=2 and irLock=1, this means the weapon will lock on and the ammo will home in on target.
irTarget is for cfgVehicle and determines if a vehicle can be targeted (using canLock and irLock).
irTarget=1 means a vehicle can be targeted and "locked onto", irTarget=0 means you will not be able to lock onto that vehicle.
(same with laserTarget I think)