Hysteresis in CMOS Schmitt Trigger
This circuit is typically used to change the trip point of a standard inverter. If you look closely you'll find that the size of the feedback MOSFETs play a major role in deciding the Vin at which the output will switch from low to high or vice-versa. 

Vih, is the input voltage at which the output switches from high to low. The source voltage of N2 is reponsible for that. Now the N2 source voltage depends on the output voltage, size of the feedback nmos. The more the width of feedback nmos N3, lower will be the Vih. Same holds true for the low to high condition in the Pmos side (which affects the Vil).