A Mantra cry is a cry that a baby lets out when tired that, literally, sounds like a mantra - monotone & rhythmic, over and over again. It's not the same as a real "i need your attention" cry, and it's best to step back if baby is doing it (the Mantra cry), as s/he may be trying to soothe themselves to sleep. Each baby's mantra cry is different.
That's why it's important to be able to distinguish between the two cries -
if you jump in during a mantra cry, you may be interfering and doing more harm than good; but if you ignore a genuine cry, it will only escalate and make it harder to calm baby.