If you find
There are good genes in most collections of animals or plants but they are not evenly distributed and some individuals make better seedstock as a result
AND
Keeping the best genes to breed with the best genes is difficult unless the individuals bearing the genes have been identified
Then,
A top mob is just the pick of the gene pool kept for mating to each other to create more animals (or plants but it usually works best with animals) of that quality or better.
It is a good way to increase the overall quality of the gene pool by having some higher quality animals to supply better genes into the pool rather than letting animals mate randomly because that can lead to good genes being mixed with lousy ones.
By selecting out and maintaining a top mob, you can choose which to mate to which and then can let those genes cascade into the rest of your seedstock.
Having a top mob means you also have a non-top mob.
The best way to boost the quality of genes overall is to have a top mob, a middle mob and a bottom mob (culls).
When times are good, all members might get mated. When times are bad, the bottom mob finds itself on the way to the saleyards.