To put yourself in the position of the other. You forget selflessly about yourself in any respect, for the sake of understanding the other person. It is putting yourself in the other's skin. It is not about agreeing and becoming like the other person, but feel what he or she feels to better understand the state of the other.
Compassion is to accept the other as an entire being, his Ying and Yang side, understand the good and the bad part of each person. See the person as a balance.
When the black part disturbs you, watch whether you can trace the same behavior in you. Reflect, breath it in and analyze. Is it your own energy and pattern?
Are you sleeping well, are you making a lunch nap? Is it about anger, grief, lack of love or emotion you cannot explain?
Be calm and settled to feel what you feel.