Having a dedicated IP basically means that you will get access to port
25565. This port is the default port of Minecraft, so when using this you will not need to specify a port when connecting to your server. If you do not have a dedicated IP, it is likely that someone else is running a server on the same IP on port 25565.
For example, let's say you have a server without a dedicated IP and your main port is 25566. In this case you will need to connect to your server with
example.com:25566 (if you have a domain!).
If you have a server with a dedicated IP your main port will be 25565. In this case, when connecting to your server you can simply use the IP of your server; e.g.
127.0.0.1 and will not need to specify a port. For a domain you can simply connect with
To purchase a domain, click the link here.
If you are the owner of a domain name you'd like to use with your server, you could also use an SRV DNS Record. This record is able to 'mask' the port of your server, thus not needing to specify a port when connecting.