What is CH4 and what does it stand for?

CH4 is the chemical formula for methane.
Methane is normally found as a gas, and is the simplest hydrocarbon and one of the most common molecules created by living organisms. It's also called natural gas and can be created when organic materials decay, such as in fossil or oil deposits, or landfills. Methane is also a powerful greenhouse gas.
The "CH4" name is describing the atoms the methane molecule is composed of. C is for carbon, which is usually found at the center of such molecules and is unique in that it has the ability to form four strong bonds, which few other common elements can do. The "H4" represents four atoms of hydrogen, each of which is equally spaced around the carbon in three dimensions, so that the actual molecule looks something like a pyramid.
https://www.epa.gov/ghgemissions/overview-greenhouse-gases