You have seen methane burn: it reacts with oxygen and ends up as carbon dioxide and water. Burning is one kind of oxidation. For a carbon compound, oxidation means the molecule gains oxygen atoms or loses hydrogen atoms.
Chemists follow it by watching one carbon atom and its four hands. Each step of oxidation swaps one of the carbon's bonds to hydrogen for a bond to oxygen.
Methane gains an oxygen atom and becomes methanol. Methanol then loses two hydrogen atoms, one from its carbon and one from its -OH, and the carbon and the oxygen share a second bond: a C=O. Gaining oxygen and losing hydrogen both take the carbon one step further down the ladder. Methanoic acid, one step from the end, is the sting of some ants.
The reverse, losing oxygen or gaining hydrogen, is called reduction. Adding hydrogen across the C=C of ethene to make ethane, an addition you have met, is a reduction.
Burning goes all the way
Carbon dioxide is the end of the ladder: its carbon has all four bonds to oxygen and no hydrogen left. When a fuel burns in plenty of air, every carbon atom is oxidised all the way to carbon dioxide, and every hydrogen atom ends up in water. Lesson 18 looks at fuels and the energy they give.
Gentler oxidation stops partway down. The molecules in between are the alcohols, aldehydes and acids you already know, and the rest of this lesson finds them in wine, apples and cooking oil.