Full tea is a type of afternoon tea meal. It is what most Americans think of when they hear the phrase "high tea," but it is actually low tea / afternoon tea.
As you might guess from the name, full tea is heavier than light tea. It is typically made up of tea, various sweets and various "savories" (savory/salty foods).
Sweets like scones are popular for full tea, as are other light tea foods like Madeleines and Battenburg cakes.
Usually, savories in full tea include finger sandwiches, but they may also include soups, quiches or lighter savory snacks, like seasoned nuts or cheese and crackers.
Full tea is often (but not always) served on a three-tiered tray. Typically, the bottom tier will hold savories (like finger sandwiches) and the higher tiers will hold two types of sweets (like scones and petits fours). It is usually recommended that you begin with the bottom tier and work your way up.