Tonglen is the practice of ‘giving’ or ‘sending’ (tong) happiness and wellbeing and ‘receiving’ or ‘taking’ (len) pain and suffering. It is part of the instructions on ‘mind training’ (Tib. lojong) brought to Tibet by Lord Atisha, and is specifically related to relative bodhichitta.
Put very simply, the Tonglen practice of giving and receiving is to take on the suffering and pain of others, and give them your happiness, well-being, and peace of mind. Tonglen uses the medium of the breath. As Geshe Chekhawa wrote: “Giving and receiving should be practiced alternately. This alternation should be placed on the medium of the breath.”