Your DHCP IP address lease has a finite life, ranging from a few hours to a few days, depending on the DHCP server and its configuration.
The DHCP server allocates addresses from a finite pool. In most home environments the pool is far, far larger than will ever be required, so it is largely irrelevant.
However, in the corporate environment, the pool would quickly become exhausted if addresses were just handed out and never returned (by the lease expiring).
If the client computer does not renew its lease within the given lease period, the address is considered to be unused and is returned to the pool.
That's a very minimalist description, I could go into far more detail than you would believe possible!
SD