IDK stands for "I don't know this person". LinkedIn has a feature where users have the ability to accept or decline an invitation. Meaning, users have complete control of the people they want to add to their networks. If a user receives an IDK response from another user, he/she will not be able to send another request to the same person in the future. If there is an excessive IDK's received by the user, he/she might get restricted from sending requests to other users.