As participant of the net, you have an IP address, under that you appear. If you do not use a dedicated line to connect to the internet, you internet service provider assigns an IP address to you, oncde you dial in. The provider has a pool of leased addresses, and selects a cuurently free one.
Hence, the address can change depending on availability.
If you connect via dial up with a modem, it is most likely to get another address on every connect.
If you use DSL, especially with a flatrate, the IP adress should change less often. A change is possible during the disconnect, that a provider forces regularly – usually every 24 hours – or at expiration of the lease-time. This lease time describes, how long the address will remain valid. Afterwards it is replaced by a new one.
Of more importance are these address changes, if you want to steadily appear as a communication partner. An example would be the operation of a webserver. It can not be addressed with its IP address, if the address keeps changing. This problem is solved with dynamic DNS.