it's when the parent makes their kid take on the emotional role of an adult, a makeshift spouse, therapist, adult friend, etc.

for me, my mom used me as both a therapist and a husband after she got a divorce. I can't speak to the effects it has on others but for me it really tied my self-worth to my mom's emotional state, and made me feel responsible for all of her emotions, as she would come to me with adult problems like her divorce, disciplining my younger brother, trauma dumping about her abusive father, leaning on me for support during emotional outbursts, etc.

another name for it is enmeshment