Abby.... in its simplest term attachment means to care about something or someone. to like something or someone.

human beings naturally attach their self...

example

some people have a favorite author, actress or other famous person.
some people have a favorite object or food that they like.
a mother likes and cares about her babies
a wife like and cares about their wife/ husband

a person in therapy can like and care about their therapist or other mental health treatment provider.

sometimes these attachments will interfere with a persons life

example sometimes a child will panic if their favorite blanket or other object that they are attached to gets lost.

in therapy sometimes caring too much about a therapist may end up in a situation where the one in therapy feels if they say something that will hurt the therapist or be too much of a problem for a therapist to handle

and on the other side of things a therapist who cares for their client can sometimes care too much to where it interferes with getting to the root of the problems because the therapist doesnt want to do their job of having the client face their problems.

sometimes attachments in therapy between a therapist and client can end up breaking laws or ethics if the attachments move in that direction.

for example here in my location it is against the ethics of performing therapy if the caring and liking each other end up into having a sexual or dating relationship while they are client and therapist.

attachment isnt something thats made up, its the word used to describe liking and caring about each other. but like with anything else there are laws and ethics around this natural issue to protect people who have mental illness.