Attachment is a fancy word that means that something is stuck or connected to something else.
In law, attachment means that a court or judge can take control of someone's property or money as a way to make sure that the person they owe money to is paid back. This is like when a teacher takes away a toy from a kid who is not sharing it with others.
Let's say your mom owes money to a person, but she doesn't pay it back. The person she owes the money to can ask the court to attach or take control of something that belongs to your mom, like her car or her bank account, until she pays the money back.
Attachment is a serious thing and it is important for people to pay their bills and debts on time so they don't get into trouble with the law or lose their things.