A site token is a wearable receipt for an SCA event. The trinket is a visible indication that you have paid the entrance fees for an SCA event and are therefore attending "legally". In mundane terms, think of it as similar to the bracelet one might get for entrance to a concert or amusement park. Sometimes, different tokens will denote different pay levels at an event, such as separate adult and children's fees or to mark who is on-board for a feast.
Site tokens can be as simple as a piece of braided yarn or a string necklace with some beads. They can also be more elaborate and customized for a particular event. Cast pewter event tokens have become popular in Meridies and are used sometimes even for local events. Pennsic is known for its annual custom-made site tokens. In their more elaborate form, site tokens can be a collectable souvenir of an event.