Question

what do you mean by chewing the cud

Answer 1

Cud means partially digested food in ruminants (like cows buffaloes)
When the ruminants bring back the food which they swallowed and chew it well this is called chewing the cud.

Hope this helps! 
@crystinia

Answer 2

Cud is the digested food and chewing that cud is in ruminants these animals do not chew the food instantly they take that food in their pounch and take out in mouth and keep chewing as you seen cows keep chewing this process is known to be chewing the cud