A dynamic IP address is an IP address that can change every time you connect to the Internet. A dynamic IP address is generally assigned by a router that has been configured to distribute IP addresses. A network that is configured to assign dynamic IP addresses does so every time a network device is connected, without any setup required by a person.
Dynamic IP addresses expire after a certain period of time has elapsed, usually configured when the network is setup. It is common to have dynamic IP addresses expire after a few days. When a dynamic IP address expires, a router will assign a new dynamic IP address. This can be the same IP address the network device previously had, or a new IP address. For this reason it is preferable for certain network devices to have static IP addresses assigned by a network administrator.