"정" is a really complicated concept.
there is no english word equivalent to it, but the closest to its meaning, I think it would be affection.
the feeling or connection that you feel toward something or somebody.
there are two kinds of jeong, goeun-jeong and mieunjeong.
goeun-jeong is a love-love affection, like husband-wife relation.
it's just, to make it simple, love.
mieun-jeong is a love-hate affection.
for example you have a life-long bestfriend that you love so much, you'd sacrifice your life for him if you have to.
yet, he's such an annoying guy you feel like killing him sometimes.
this is goeun-jeong.
and jeong is not just about people-people relationship.
you could also feel it toward something.
for example, the ring your grandmother wore at her wedding that she gave to you.
your grandma's gone now and you love this ring with all your body and soul.
that's also jeong.