The Grains controls let you set the speed at which grains are generated automatically, how long each grain is, what kind of amplitude envelope will be applied to it, and how loud it will be.
Each slider has a random range associated with it, which is displayed as a semitransparent red bar surrounding its control knob. For detailed information about how these sliders work see Tutorial 1: Understanding the RTGS3 slider controls.
Density = the time difference (in milliseconds) between the onset of one grain and the onset of the next grain
Max. Density = sets the maximum value which can be entered using the Density slider
Length = the total duration (in milliseconds) of each grain read from the source sound in the Buffer
Max. Length = sets the maximum value which can be entered using the Length slider
Amplitude = the difference (in percent) between the amplitude of the source sound in the Buffer and the amplitude of each grain
Amp. Curve = sets whether the Amplitude slider uses a logarithmic or exponential curve when applying amplitude scaling
Grains On/Off = this toggle starts and stops automatic grain generation
Generate a Grain = clicking this button will generate a single grain using the current settings (useful for checking the effectiveness of your settings)
Backwards Grains = when this checkbox is active, grains are read from the Buffer backwards, starting from the position set in the Buffers Position slider
Envelope = selects the type of amplitude envelope to apply to each grain. The 2D slider lets you control the shape of that envelope.