What does this mean?
Chickens have a limited time to "earn" their way by laying eggs. When they reach two or three years, they cost more in food than they produce in eggs. You have to replace them with younger chickens (bought or raised). The older, non-productive ones become soup or stew.
It means that when new chickens are brought in, the old chickens get eaten. The idea of batch just means that a lot of new chickens are obtained at basically the same time, as a group, rather than in small numbers every day or few, forever. Batch (group) rather than continuous (frequent individual) replacement.