Definition of runtime
In programming, RUNTIME is the time interval from the moment the operating system starts executing the instructions of a given program until the end of its execution, either because the program was successfully completed or because it was terminated by the operating system due to a runtime failure.
Runtime in Unity
When we are developing a game or application in Unity, the runtime of our program starts when we press the Play button until we press Stop and also when we make a compilation for windows for example, the runtime starts from the moment we run the application until it is finished.
It is important to understand this concept of runtime in Unity because we have to be able to handle situations that will occur during the execution of the program, for example enemies that appear in the middle of the game, these enemies will need to be provided with certain information that was not possible to give them at the time of game development simply because they did not exist at that time, so the person responsible for creating these enemies must also give them the information they need, for example the reference of the player they have to attack.