Plopping is a technique used by people with curly hair, and it's essentially code for drying your curls with a cotton t-shirt as opposed to simply wrapping it up in a towel. It helps to reduce frizz after washing while also leaving you with bouncier, tighter curls.
There's really not much to the process: all you have to do is apply leave-in products to your hair as you normally would after washing and then wrap your hair in a t-shirt until it's dry. Check out the detailed tutorial above if you're more of a visual learner.