A storm needs energy to grow, just like the light in your living room can't turn on without energy.
In the weather world, we look at CAPE which means convective available potential energy. This is the energy that could be used to build a thunderstorm. To analyze the weather we look at an area of air at the surface called an air parcel. An air parcel that's warmer than the surrounding air rises. If it can rise all the way throughout the atmosphere thunderstorms can form.
The more CAPE, typically leads to a stronger storm.
Sometimes we have a layer of warm air above the surface also called an inversion. If the parcel is not warmer than the environment, it can't longer rise and storms can't form. This is known as the CAP. You can have a lot of CAPE, but if the CAP is there you won't see they towering cumulus clouds that eventually become cumulonimbus storm clouds.