Have you ever seen fire? Fire is hot, isn't it? The heat comes from a chemical reaction called burning. Just like how we need oxygen to breathe, fire needs oxygen too. When we burn something, it reacts with oxygen to make heat, light, and other gases.
Now imagine we have a container that is perfectly insulated, so no heat can escape. We put some gas inside the container and mix it with oxygen. We light the mixture on fire and watch it burn. Because the container is insulated, none of the heat can escape, so the fire gets hotter and hotter until it reaches its peak temperature.
This peak temperature is called the adiabatic flame temperature. It's the highest temperature the fire can reach without any heat escaping from the container. The temperature depends on the type of gas and how much oxygen we mix with it. Different gases have different adiabatic flame temperatures, and the amount of oxygen we mix with the gas affects the temperature too.
So adiabatic flame temperature is simply the highest temperature fire can reach without heat escaping from the container. It depends on the type of gas we use and how much oxygen we mix with it.