DDNS is a service that maps internet domain names to IP addresses and allow you to access your home computer from anywhere in the world.
DDNS serves a similar purpose to Internet's Domain Name System (DNS) in such that DDNS lets you to host a web or VPN server advertise a public name instead of numbers.
Further, unlike DNS that only works with static IP addresses, DDNS is designed to also support dynamic (changing) IP addresses, such as those assigned by a DHCP server. That makes DDNS a good fit for home networks, which normally receive dynamic public IP addresses from their internet provider. The result is that if your IP address changes, your domain name will adapt to the new IP address.
To use DDNS on the V4, you have to sign up at http://freedns.afraid.org/signup/?plan=starter with a free starter account. Only your email is needed for verification and a confimration will be sent to your email after registering. Check your spam boxes if you do not receive the confirmation. (Though if you like the service, you can make a donation at your discretion)
After your confirmation, you can login to create your desirable domain name and obtaining the API which you will need to enter these 2 information in the V4. V4 will then periodically check if your public IP has changed and upate to freedns.afraid.org, which takes 60 minutes to effect.