The Android Developers web page on Android Application Boot explains this in three terms: “cold boot”, “warm boot”, and “warm boot”. A cold start is a state when an application that was closed previously is launched from scratch. As Google notes, a cold boot requires more time and resources than in other states because it has to handle more processes when starting an app. A warm start, on the other hand, means that the app is already in the phone’s memory – i.e. you can access it from the multitasking menu – and the Android device should only undergo a “subset of processes” vs. cold start. Finally, a warm start means that a user simply brings a recently used application to the foreground. During a warm boot, Android needs to perform minimal activity to keep used apps alive.
An easier way to understand this is to compare it to your car on a heavy snowfall day. Starting your vehicle in sub-zero temperatures can be a tall order. However, if you’ve been using the car for a while and only stopped for a quick coffee or at a traffic stop, you can start your vehicle fairly quickly. And finally, if your car is actively driving or you have a problem like releasing the clutch too abruptly, starting the car will be effortless.