A mantra is something that you would just repeat over and over, for example, the syllable that everyone seems to be familiar with and popular one like Om, you can say Om over and over.

曼怛罗就是咒语,你一遍遍重复的内容，比如，大家都熟悉的音节,或唵这种众所周知的，你可以重复念唵音。

普林斯顿公开课 - 人性课程节选