The word mantra is said to come from a root meaning “that which protects the mind.” In Buddhist meditation, many things can be used as objects of concentration — as “mind protectors.”
The breath is used in anapanasati, or mindfulness of breathing meditation. The sensations of walking are used in walking meditation. The emotions are used as a focus in metta bhavana (development of lovingkindness). And visual images are used in visualization. Mantras are sounds — words or phrases — that are used as an object of concentration.