Trunks are essentially the same thing as saying telephone lines.
Why are trunks in your call centre important?
If you dont have enough trunks in your call centre you will not have any ability to place customers on hold as your ACD needs trunks to hold calls in a queue.
A lack of trunks can also result in customers being unable to access your call centre at all.
If there are not trunks left, a customer will receive a busy/engaged signal.
Newer cloud solutions for call centres also need trunks, however, they rely on SIP trunks which are far less costly to purchase and maintain.