These terms mean you don't know what is YOU and what is HER. You don't have a sense of a separate identity. When she hits her thumb with a hammer, you shout "ouch!" When she is hurting, you feel you must fix it.
What we are striving for is differentiation... You're here. She's over there. You two meet, touch, interact, share, communicate, etc. but you don't lose sight of the fact that you are two different people (except maybe during really GREAT SEX for a few seconds). You don't do her work, she doesn't do your work.
This is a challenge. And we're all under the sway of that BS myth that says we must "become one." Once you left your mother's womb, that was the last time you should be "one" with anyone-- except for a few seconds during really GREAT SEX and other peak times... there are those moments... I'm not making fun of them. But 99.46274% of the time you are separate and feel separate. When you're separate then you can CHOOSE to be together. When you're enmeshed/fused you're stuck in quicksand, you have your feet nailed to the floor, and you have given up your powers of choice.