During an (Celebrity-)-Endorsement, a famous person reccomends a product or a brand to their fans. For example, many manufactures of dietary supplements are succesfully hiring famous fitness-models on Instagram or Youtube to endorse them. But other channels with a huge strength of reach like Twitter and Facebook can be used for endorsements as well.
Endorsements are very effective as a near-term advertising effort. If the product and the endorser have a perfect "fit" , the endorsement can be a relatively cheap advertising method.