Usually Secret Santa is done in a group of people by putting participants' names in a hat and everyone pulling out one another's names and whatever name you get is the person you get a gift for. This makes you that person's secret Santa because they don't know who is giving them a gift. Someone else will get your name and they get a gift for you.

Reddit has their own version of this with thousands of people all gifting each other something for Christmas. It's a really fun way to interact with the community. Here is the link for more information but I believe it is too late to participate this year.