Like so many other things any domain has an expiration date.
If you fail to renew your domain name before the expiration date, ALL Internet services associated with the domain name(s) will stop working, including email, email forwarding, web sites or any other applications.
Any time left until your expiration date will be retained and added to your renewal period. For example: If you have 6 months left until your domain expires and renew the domain name for 1 year, you will have a total of 1 year and 6 months.