Meaning
Han is the seed syllable, or beeja, assigned to Hanuman; Hanumate is his name in the dative case, and namah means an offering of respect. The whole reads, roughly, Om, to Hanuman I bow. Tradition remembers him as the vanara devotee of Ram whose strength was inseparable from his service, so the seed carries both qualities together.
हनुमान जी का बीज मन्त्र।