Che cosa significa Devi ringraziare la signora Maria per questo favore.

Hai ricevuto i soldi?
Sì, li ho ricevuti tutti.(why is it here "ricevuti", not "ricevuto"?) ?

Vedi una traduzione

the first sentence means "You have to thank Mrs. Maria for this favor."
the second one "Did you get the money?"
the last one "Yes, I got them all"

the particle "li" is an object pronoun that substitutes "soldi". The verb is compound (ho ricevuto = Indicativo passato prossimo) where "ricevuto" is the past participle.
When the compound verb is preceded by an object complement, you must concord the past participle in gender and number of the object complement.
Another example:

Hai conosciuto le mie sorelle? (Did you meet my sisters?)
No, non le(=sorelle) ho conosciute (No, I didn't meet them)