What it is
Mañjarī means a flower-bud, the small unopened blossom on the creeper. In the Vraja imagery, the eight principal sakhīs of Rādhā each have several younger maidservants, pre-pubescent girls or just barely past it, who run their errands and serve in the kunja. Below the eight sakhīs serve the eight mañjarīs, the closest, most intimate attendants of the divine couple in the secret nikuñja.
The mañjarī's distinctive feature is this: she is not in love with Krishna in her own right. She is not a sakhī who, by her own age and station, has her own meeting with him. She is small, young, secondary; her whole devotional life is given to Rādhā. When Rādhā meets Krishna, the mañjarī arranges the kunja, places the flowers, holds the betel. When they are alone together inside, the mañjarī waits outside with the others, and her joy is total because her mistress's joy is total.
The Sanskrit theological term for this is para-duḥkha-duḥkhi: pained by another's pain, pleased by another's pleasure. The mañjarī's joy and grief are not her own. They have been completely identified with Rādhā's. This is held by the tradition to be the absolute end of self-interest in love, and therefore the deepest possible relationship with Krishna, accessible only by going around him to her.