It is a mixture of what we now think of as bowing and curtseying. There are a few variations, but a usual component is that both knees must be bent and there is no requirement that the upper body leans forward. The position and movements of feet seem to differ.
The word is not correctly spelled with an a, though a misspelling with an a is often seen, and too often pronounced. This is apparently an attempt—by people who know no French—to simulate or approximate the French word révérence.