At its core, it means loving-kindness, but it runs much deeper than that. It is the well from which our kindness flows, giving and wanting nothing in return.
What makes metta so interesting is that it is not about self-sacrifice. It teaches us that we should not always put others above ourselves. That we must ensure to take time to appreciate us as well, to give ourselves a bit of kindness too.
So take off your shoes, get comfy, and press that play button.