Compassion is pro social, animated by action, and a deep desire to help. It's very different from empathy (I feel your pain) and rather, at least in the way we are talking about the practice, is loving kindness in the presence of suffering.  

(where the pain of another becomes the object of loving kindness, with compassion being the "fruit" of the practice)

Bring to mind someone who is suffering.  Generate the well wishing for them to be relieved of their suffering.  Picture them on the very best day of their life.  Imagine them happy, healthy, smiling and free.  Imagine them being loved.  

If you could give them this experience, this moment, you would.  

Animated by action, engaged and attentive, and in service of real people in the real world, this is where the practice of compassion becomes portable, and powerful, both for yourself and others.