Internet Service Providers assign dynamic IP addresses using the Dynamic Host Configuration Protocol. This was originally designed for dial-up users who wouldn't be connected all the time, allowing ISPs to reuse a limited number of IP addresses. It's still commonly used because it's the default and it automatically assigns an IP that's not already in use by another computer. Most people won't need a static address. You'd need it if you want to run your own webserver, for example.
You can ask your ISP for a static address. IPv4 addresses are scarce, but you can probably get an IPv6 address for free.
You can also set a fixed IP address for your computer in your operating system, but that will only be used within your home network.