Mentoring is a relationship between two people, designed to build confidence and support the mentee so they are able to take control of their own development and work.
The mentor is experienced and knowledgeable, a trusted advisor, but the responsibility for making things happen and putting plans into action lies primarily with the mentee. The mentor is NOT normally the mentees line manager although the line manager might including mentoring in their formal and informal meetings depending on their job roles so it is a useful skill for any manager to have. The mentor might be from a different department, or even a different company and is quite often voluntary.
The Mentee might increase their self-confidence and motivation, feel supported and also challenged to develop. They can take the opportunity to reflect and observe others, to develop a relationship with a senior colleague which might be a sounding board or an opportunity to think differently.
The Mentor might find it satisfying seeing others develop, have increased recognition from peers, be challenged and stimulated, be able to see future potential for the person there are mentoring, learn more about themselves and develop their own skills.