The "los" in this case means "them". It's a plural direct object pronoun. We go with the masculine form because the singular el papá ties together with the plural los papás
The sentence "Manuela quiere a sus papás. Manuela <u> los </u> quiere" translates to "Manuela loves her parents. Manuela loves them". You could also make an argument to replace "loves" with "wants", and mean the same thing (more or less). This is because the base verb querer means "to want" and it also means "to love", among other slightly similar/related meanings. So it all depends on context pretty much.
