Function of MOSFET based circuit
This circuit basically operates as a switch, allowing current to flow through \$Q_1\$ depending on the state of \$Q_2\$.

I assume that you have some logic driving the gate of the mosfet \$Q_2\$, by either pulling it to ground (turning it off), or pulling it high via resistor \$R_1\$.

If the gate of \$Q_2\$ is high, it is ON, meaning that the gate of \$Q_1\$ is pulled to ground via \$R_3\$, which in turn turns it (\$Q_1\$) ON as its \$V_{GS} &lt; V_{TH}\$. 

If the gate of \$Q_2\$ is low, it is OFF, meaning that the gate of \$Q_1\$ is pulled to its \$V_S\$ via \$R_2\$ and \$R_3\$, which in turn turns it (\$Q_1\$) OFF as its \$V_{GS} &gt; V_{TH}\$. 

Sometimes it is necessary to turn off/on the inline MOSFET \$Q_1\$ a bit more slowly, (e.g. to improve EMC). However, it comes with the burden of increasing the power dissipation of the transistor since it operates longer in the linear region (large \$R_{ds,on}\$). This slower switching is achieved by adding the gate resistor \$R_3\$ which slows down the charging and discharing of the \$Q_1\$'s gate capacitance. 

The resistor \$R_2\$ is necessary to limit the current through \$Q_2\$ when it is ON.

The \$C_1\$ adds some filtering to the \$V_{GS}\$ of \$Q_1\$, attenuating high spikes from the supply / source voltage. Furthermore, it also slows down even more the turning on/off time of the MOSFET (\$Q_1\$).