The attenuation/gain of the eq.

The centre frequency (ie. point of most/least attenuation).

Beware of high values for Frequency and Bandwidth, if the high pitch (Frequency * 2$^{Bandwidth}$) goes over half the sample rate you will get aliasing.

Note: if your host offers you a frequency range between 0 and 0.4 then it's not rendering the input parameter correctly, the input frequency will actually be that number multiplied by the sample rate (e.g. 44.1kHz).

The pitch difference from the centre before the attenuation has reached half the gain.