Tonglen means something like 'giving and taking'. What you take in is the suffering of beings. Then 'you' transform that into loving-kindness and compassion and send it back out to beings. Easy peasy, then.
I've put 'you' into inverted commas because 'you' here does not imply any egoistic 'Hey, here I am saving the world' sort of approach. It's much more humble that that.
The wish to do all this giving and taking stems from the the point of view of a bodhisattva, someone who has vowed to live and act so as to save all beings from suffering. Easy peasy, again then. The context does give him or her many, many lifetimes to achieve this task, so I suppose that helps a bit.
Whether or not you take this ideal of the bodhisattva literally, and whether or not you believe you have lots of unlived lives yet to come, this tonglen practice is a beautiful, demanding and fertile meditation.
It's a way of connecting with suffering wherever we are and whatever the circumstances of our life. And it's a way of overcoming any fear of suffering we may hold. It's also a way of becoming less dependent on personal happiness, or to put it another less bound into chasing after happiness. That's not to say there is anything at all wrong with being happy. But there is quite a lot wrong, or at least unhelpful, about spending one's life chasing after it.
You may have noticed that happiness doesn't last.
Here's how to do the practice.....