Every note played in Reason, has a certain velocity. One can think of velocity as "how hard the piano key is pressed". Actually, with most MIDI-controllers this is the case: playing it softly will send notes with a lower velocity to Reason, while hitting it harder will send higher-velocity notes to Reason.

The obvious effect to be coupled to velocity is the volume of playback. This is what you have on a real piano: when a note is played softly, it plays back silently. You hit that piano key really hard, and the note(s) you hit sound louder.

But velocity isn't limited to the playback volume alone. On all sound-producing devices in Reason (like samplers and synths), one can assign all sorts of things to be linked to the velocity of the notes. Like the sample's starting point, or the filter cut-off frequency. This is what you can tweak with the little knobs in the velocity section of the device. When all those knobs point at 12 o'clock, all notes played will sound the same, no matter their velocity.

But, if you turn a velocity knob - like the volume or f. freq knob - to the right a bit, it will mean that notes with higher velocity will be played back with a higher volume or filter cutoff frequency. When the velocity button for a certain quality is turned to the left, the corresponding quality will decrease as velocity increases.

It's simpler than it seems, really.

Note that on the ReDrum, velocity corresponds to the colour of the notes you put in a pattern: yellow means low velocity, orange means medium velocity, and red means high velocity. The standard setting is that these correspond with playback volume (Red is played back the loudest), but if you point the Velocity knob from a certain sample in ReDrum to 12 o'clock, yellow, orange and red will all be played back at the same volume.