A dynamic IP address is an IP address that is temporarily assigned to a device, and can therefore change over time.
Unlike a static-ip-address that is permanently assigned to a device, a dynamic IP address is only temporarily assigned.
While this is usually about a consumer router's WAN (internet-facing) IP assigned by the ISP, this could also be about IPs within a LAN temporarily leased to a device using DHCP. In some cases, DHCP will cause the same device to receive the same IP if it is regularly connected to the router, therefore making the dynamic IP semi-permanent, although strictly speaking not static and therefore open to change.