Ok, singular and plural, But.....the problem to undestand the Spanish's use is the different structure in English and in Spanish.
In English, the subject is the person who likes. I like something.
But in Spanish the subject is the think: Algo (something, subject) gusta a mí (person, object)....Really we say : Algo me gusta.
The concordance is subject-verb:
Singular: La manzana me gusta (subject: la manzana)
Plural: Las manzanas me gustan (subject: las manzanas) (gustan = plural because manzanas = plural).