It’s a statement made by the seller or manufacturer of a product saying that it will perform in the way specified.
Products can have problems either upon purchase or after a certain amount of time. Defects sometimes are not apparent until you are using the product. A warranty is a sign that the company has confidence in its products and that will last longer.
Consumers will be re-assured by a warranty on your products and services, so if you have one make it clear on your website. If you don’t have one, it’s certainly something to consider.