Direct object pronouns replace a noun that directly receives the verb's action, with no preposition in between: me/m' (me), te/t' (you), le/la/l' (him/her/it), nous (us), vous (you), les (them). Unlike English, they're placed before the conjugated verb, not after: je le vois (I see him/it), not je vois le.
Le, la, and les replace a specific noun already mentioned or clear from context — tu as les billets? Oui, je les ai (do you have the tickets? Yes, I have them). Le can also stand in for an entire idea or a previously stated adjective: est-elle fatiguée? Oui, elle l'est (is she tired? Yes, she is).