A mentor is a person who is considered a veteran in BDSM as a lifestyle, has agreed to take on the guidance (mentoring) of a pupil into the lifestyle and has earned the trust of the pupil. The mentor will answer questions, help the pupil find resources, console them on possible internal struggles or give them resources for such help. A mentor will be sure to listena, understand and answer questions in regards to BDSM as well as help guide the pupil towards beneficial information and experiences.
A mentor ideally has a lot of real world experience with BDSM and can be an s-type or D-type though generally it is best to learn from someone who is walking a similar path to the pupil. The role of a mentor does not equate to being someone's dominant or submissive, play partner, trainer, or sex partner. It is important to determine that attaining one of these positions is not a hidden motive of a potential mentor as it is not entirely uncommon to have predatory individuals offer to mentor new explorers of BDSM as a ruse to gain access to the uneducated and vulnerable, so be sure the mentor you choose is someone that you can trust not to take advantage of you.
It is sometimes discussed that it is better to learn from the entirety of the BDSM community rather than a single individual to gain many perspectives and also prevents one from being stuck learning under one person that could potentially mean the pupil harm. This application has some benefits as well as flaws. While learning from the entire community is a great long term goal, up front many new BDSM explorers are unable to take it so much information all at once and will suffer an information overload. This will not only cause undue stress, but also will not allow them to have much guidance and a personalized experience with a mentor which can be beneficial during initial feelings of discomfort about BDSM activities and fantasies; it is much easier to get close enough to discuss these things at first with one person than it is to discuss them with many people. Further, it is easier to guide someone during initial stages if one person is doing the guiding rather than many which will again, cause information overaload. Ultimately it is vastly important that you develop a strong trust bond with your mentor if you should choose to have one, and that at some point you will want to learn from many others. Whether you choose to learn under one person or many to start is largely a matter of personal choice for preferred methods of learning and there are merits and flaws to both methods.