What is Plato's 'theory of recollection'?

Plato's 'theory of recollection' goes back to an idea he elaborates in his dialogue the 'Meno'. Essentially, Plato believed that our soul had a pre-existing state where it enjoyed knowledge of the forms. However, in its present state of being embodied it loses such knowledge, and all learning refers to the process of recollecting such knowledge.