Indirect Object Pronouns are used as the indirect objects of verbs. As with direct object pronouns, they are required when the indirect object is a pronoun (in contrast to a noun) but a prepositional phrase may be added for clarification or emphasis.

¿Me hablas? - Are you talking to me?
¿Me hablas a mí? - Are you talking to me? [a mí added for emphasis]

Le dije la verdad. - I told him/her/you the truth.
Le dije la verdad a ella. - I told her the truth. [A ella added for clarification since le can mean “him” or “you”.]

Since these are with-verb pronouns they cannot be used if the verb is only implied. The preposition a plus prepositional object pronouns are used in such cases:

¿A quién hablas? ¿A mí? - Who are talking to? (To) Me?
Sí, a tí. - Yes, (to) you.

In contrast to the situation with direct object pronouns, indirect object pronouns are normally used even when a noun is expressed as the indirect object:

Veo a Carlos. - I see Carlos. [Carlos is the direct object; lo is not added]
Le mando un regalo a Carlos. - I'm sending a present to Carlos.
Les escribo a todos. - I write (to) everyone.

When parts of the body and articles of clothing appear as the direct object in Spanish, normally the definite article is used instead of the possessive adjective (my, your, his); an indirect object pronoun is used to indicate the person involved:

Me pongo los zapatos. - I put on my shoes.
Ella se cortó el dedo. - She cut her finger.
Les duelen los brazos. - Their arms hurt.