Japa is the repetition of a sacred name, mantra or prayer as an act of remembrance and spiritual attention. It can be part of everyday devotion or a more formally guided discipline. People may use audible recitation, quiet murmuring or mental repetition, depending on tradition and circumstance. In many devotional settings the purpose is not merely accumulating repetitions but cultivating attention, devotion and remembrance.
The Yoga Sūtra 1.28 is a useful textual reference: it connects repetition of the sacred syllable discussed in the preceding sūtra with reflection on its meaning. That does not establish one compulsory practice for all Hindu traditions. The eligibility and discipline of a particular mantra depend on the mantra and teaching context. This FAQ explains the concept, not a prescribed japa regimen.