Skill Endorsement:

Once you've added a skill to your profile, your skills can be validated by 1st-degree connections to reinforce their weighting. These are called skill endorsements, which are different than recommendations. When a connection endorses your skills, it contributes to the strength of your profile, and increases the likelihood that you'll be discovered for opportunities related to the skills you possess.

You don't need to request a skill endorsement in order to receive one. By default, you'll receive a notification when a connection endorses one of your skills. However, you can manage the frequency of the skill endorsement notifications