The objective of handicap races is to even out the spread of ability of athletes to give everyone a chance of being first across the line and hence 'win' the race.
How it works : The handicap is given by the race handicapper who judges athletes according to past and present performance. Thus in a 5km handicap race featuring two athletes where Runner A is capable of running 23 minutes and Runner B 20 minutes, the handicapper would apply a 3 minute handicap to runner B. Runner A would go off on 'scratch' i.e. from when the race is started, Runner B would start after 3 minutes have passed.
If both athletes perform to expectation then both runners cross the finish line at roughly the same time. As a competitor the races are quite exciting, as the slower athletes try to avoid being caught, whilst the faster athletes try to catch those who started ahead of them. Naturally all athletes bunch up at the end of races - the reverse of a conventional race.