Generally speaking, guarantees are offered by manufacturers of products. They are free of charge but legally binding under the Consumer Rights Act 2015.
In law, a guarantee is considered to be "an agreement to provide some benefit for a set period of time in the event of the goods or services being defective". For example, a vacuum-cleaner manufacturer will usually offer a guarantee with their products that, for a year or more, they will carry out free repairs for problems caused by a manufacturing defect.
It's important to remember that manufacturers' guarantees are in addition to your statutory responsibilities as a supplier under the Consumer Rights Act. A supplier cannot, for instance, refuse to deal with a customer's complaint about a faulty product simply on the grounds that the product is outside its guarantee period.
In law, a supplier is still liable for any breach of contract - for example, if the goods are not fit for their purpose, or of satisfactory quality - for a period of up to six years.