What is a PDO and how does it work?

A PDO is a preceding direct object: an object that goes with a direct verb and comes before the verb. It affects the endings of past participles in the passé composé.
For example: j'aimais les tartes, alors je les ai mangées.
So for a feminine PDO, you add an extra e and for plurals, add -s (Masc.) and add -es (fem.)