Acronym for Complementry Metal on Oxide Semiconductor.
The simplest of CMOS gates is a CMOS inverter gate. A nMOSFET and a pMOSFET are have one pin connected to either GND or VDD (respectively) and the other terminal connected as the output of the gate.
The gate pins of the MOSFETs are connected together as the input. When the Input=VDD, the pMOSFET is off and the nMOSFET is turned on. This means the output is connected to GND, which is logically opposite to VDD (the input). When the Input=GND, the pMOSFET is on and the nMOSFET is off, meaning the output is connected to VDD.