The dried leaves of the plant Camellia sinensis are used to produce tea (except for herbal teas, which some would prefer to to be called tea at all, but rather tisanes) by allowing them to rest in hot water, a process known as steeping.
In previous times, when tea was very expensive, the used tea leaves would be sold on to poorer classes for re-steeping, and were sometimes adulterated with a range of other products.
Used tea leaves can also be used for cleaning purposes.
'Tea leaf' is rhyming slang for 'thief.'