Tonglen is a Tibetan word - 'tong' means giving, and 'len' means taking.
In brief, this practice aims at taking on the suffering and pain of others, while giving them well-being.
If Buddhism asserts that it's not possible to literally take away and experience somebody else's karma, how does tonglen work?
It is said that for tonglen to be most effective, we would first have to have a deep karmic connection with the other individual, built up from this life, or a previous life.
Tonglen would then act as a condition for the surfacing and ripening of a portion of each individual's karma, which would otherwise be buried under their other ripening karma. This causes the person performing tonglen to experience something similar to what the other individual had been experiencing, while they experience well-being instead.
On this deepest level, tonglen is an incredibly powerful practice to engage in. A practice that relies not just on a deep karmic connection, but on great compassion and courage too.
There are also other, more superficial levels, of tonglen practice. Whatever level that we engage in, it's extremely beneficial, as it will dramatically weaken the self-cherishing attitude, and will undoubtedly strengthen our positive qualities, and our bodhicitta aim - the compassionate wish to attain the enlightened state of a Buddha for the benefit of all beings.