The word Tonglen, or Tong Len, is Tibetan for “Sending and Taking”, with Tong meaning “giving or sending” and Len referring to “receiving or taking”.
One could say that this is a more advanced version of Loving Kindness meditation.
With Tonglen practice you add the dimension of taking in suffering of other people on the in breath and you send out compassion, love and understanding on the outbreath.
The idea behind this is to not be in denial or resistance to the suffering or the ‘unwanted things’ in the world. Instead we acknowledge it, open our heart to it and take it in. Then in our heart we transform the suffering we have taken in and send out compassion, love, wisdom etc.
You could say it is a practise in turning around polarity. You turn ‘negative’ into ‘positive’, what is ‘unwanted’ into ‘wanted’ etc. To be able to do this you need to first drop any resistance towards one side of the polarity. By accepting the existence of that side of the polarity and then opening your heart to it you can then turn it around into the other polarity of it.
An advanced explanation with a Tonglen meditation (starts at 34 min and 34 seconds) can be found in this video: