Afterdrop is the phenomenon where your body temperature will continue to cool even after you have finished your swim and are out of the water.
It commonly occurs between 10 and 30 minutes after you have completed your swim and will cause shivering and discomfort as your internal temperature becomes very cold.
Recovery from afterdrop is a slow process as you need to rewarm your core temperature and it can be a few hours before you feel normal again.
The best way to recover from afterdrop ( in my experience ) is to get dry and dressed as quickly as possible within the 10 minute window of opportunity before afterdrop begins.
It may be tempting to have a warm shower or bath but this only warms your skin, not your core, and will delay your recovery.
It is important not to rush the recovery through afterdrop and to wrap up warm and dry and especially to wear a hat as the head loses a lot of heat. Socks may become clammy and damp if hastily put on damp feet so change into a fresh pair.