In the era of internet, we can’t run away from getting a domain name. I will tell you what is dynamic DNS and why you needed a domain name. First of all, we all would have experience the internet either from the comfort of our home or while on the road with a mobile internet subscription.
When you subscribe to a broadband plan as a home users then you will be assigned a public IP address every time we restart the router. So, whats wrong with getting a random IP address every time the router restarted? It is like having your home address changed every now and then. How would that effect the postman that is going to delivery mail/goods to you? So, back to the dynamically assigned public IP address; in order to resolve this problem there are many Dynamic DNS service providers like no-ip, dyndns, and so on (https://en.wikipedia.org/wiki/List_of_managed_DNS_providers) that you could subscribe to and solve the problem.
For a layman users, you have less choices as it will depend mostly on the router that you are using. The router would have a list of dynamic DNS providers that you could choose. Then configure your domain name with user credentials. So, it will be the router responsibility to update the Dynamic DNS provider after each re-connection or IP address change. If you are more tech savvy, then you could have some of the SBC which is up 24 x 7 which can do the job of updating the Dynamic DNS provider periodically, i.e. cronjob (a kinda unix job scheduler)
Why do you needed one?
More and more use-cases are cropping up as more commercial IoT or smart devices are available to the consumers. Things like IP based CCTV, google home mini, smart rice cooker, intelligent/smart locks, robot cleaners, fridge (coming soon), IP based switch, smart lighting, and the list goes on. So, all these gadgets of the now and future will require a digital address which is our domain name. In order for us to control them even when we are half way across the globe.