Gerund - An impersonal verb form which is used as an
adjective or adverb to express an action in progress. It is used to refer to
an action which is simultaneous with or prior to the action of the other
verb in the sentence.
Caminando por la plaza, vi a mi abuelo.
While walking in the plaza, I saw my grandfather.
Pasó todas sus vacaciones esperando una sorpresa.
He spent his entire vacation waiting for a surprise.