Activation energy is the initial energy push needed to start a chemical reaction. Think of it as rolling a boulder over a hill; you must push it to the top before it rolls down on its own. In gasoline engines, the fuel and air mixture inside the cylinder will not burn on its own, even under high pressure. The spark plug provides the heat that jumps this energy barrier, starting the combustion fire.
If the engine does not supply enough starting energy, you get a misfire. Cold engines need extra help because the cold metal walls soak up heat, raising the required starting energy. This is why fuel injection systems spray extra fuel during a cold start to make the mixture easier to ignite.
Diesel engines handle this differently. They do not use spark plugs. Instead, they compress the air inside the cylinder until it reaches over 1,000 degrees Fahrenheit. This intense heat supplies the activation energy needed to ignite the diesel fuel the instant it sprays into the chamber.