Kalyāna-mitta: 'noble or good friend', is called a senior Bhikkhu who is the mentor and friend of his pupil, wishing for his welfare and concerned with his progress, guiding his meditation; in particular, the meditation teacher kammatthānācariya is so called. For details see Vis.M III, 28,57ff. The Buddha said that noble friendship is the entire Noble life S. III, 18; XLV, 2, and he himself is the good friend par excellence: Ananda, it is owing to my being a good friend to them that living beings subject to birth are freed from birth S. III, 18.
In modern usage, it is another person interested in the Dhamma to which you can discuss the subjects of the Dhamma. Kalyana mitta groups are discussion groups for mostly lay people to discuss the Dhamma, do group study, and get advice.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.