In the sentence “She sings well,” the word “she” is a pronoun. More specifically, it is a singular, feminine, third-person personal pronoun functioning as the subject of the verb “sings.” It refers to a person without repeating or naming that person.
Pronouns help speakers and writers avoid repetition. Instead of saying “Maria sings well. Maria practices every day,” English can say “Maria sings well. She practices every day.” The pronoun’s meaning is usually understood from its antecedent, the noun or noun phrase it refers to. In the question’s sentence, “she” could refer to a previously mentioned woman or girl.
English personal pronouns change form according to grammatical role. The subject form is “she”; the object form is “her,” as in “I saw her”; the possessive forms are “her” and “hers”; and the reflexive form is “herself.” A pronoun is not a verb, adjective, or ordinary noun, although pronouns can occupy positions that noun phrases commonly occupy. In modern usage, singular “they” is also widely used when a person’s gender is unknown or when they do not identify as exclusively male or female.