If you attach something to an object, you join it or fasten it to the object.

If someone attaches himself or herself to you, they join you and stay with you, often without being invited to do so.

If people attach a quality to someone or something, or if it attaches to them, people consider that they have that quality.

If you attach conditions to something such as an agreement, you state that specific things must be done before the agreement is valid.

In computing, if you attach a file to a message that you send to someone, you send it with the message but separate from it.