These denote a family of organic compounds known as aldehydes. Each Aldehyde consists of two groups of which one is CxHyOz.
C is Carbon
H is Hydrogen
O is Oxygen
x, y, z denote the number of atoms in the group bonded together.
It is just a general formula for oxy-hydrocarbons.
For the 'x'y'z' you put in the relevent numbers.