Biasing arrangement - common gate amplifier
Biasing means you set up the operation point. Any amplifiers has different input and output impedances, gains, parasitics, etc. 

For a MOS transistor biasing means you set the gate-source voltage or the drain source current, since the device is a voltage controlled (VGS) current source (IDS). The two are strongly related by the MOS equations.

Assume that the source voltage is higher or equal than the gate. In this case the devices is closed. The current source draws charges from the source node, and since there is no source of charges, the voltage on that node will sink. This increases the VGS of the transistor as long as the current through the transistor and the current source will not be the same. This way the VGS, IDS is set and all small signal parameters are set. (But even if your amplifier works in large signal biasing - setting the operation point - is important)

RD just sets the voltage gain (gm*RD). i(t)*RD should be small enough to keep the transistor in saturation, if you want a linear amplifier. RD has nothing to do with biasing.