What is Dynamic DNS (DDNS) and how useful is it to a normal user?

I want to understand what is DDNS and how is it useful for a normal person. What does it cost, how much technical know how is required if one wants to set it up etc.

DDNS is generally useful if you have a dynamic IP address and would like to host something on your home PC/server for outside access.

Dynamic IP changes the DNS record of domain to point at your current IP address often in Real time, as you might have guessed, it requires some kind of device (preferably always on) to update your IP address in the records
So you essentially get a domain which always points at your home IP address even if it's dynamic

Some easy way to do it is by using no-ip(free) or dynDNS
One better way is to use a machine to read and update IP address using cloudflare API