Runtime environments (RTE for short) act as small operating systems and provide all the functionality necessary for a program to run. This includes interfaces to physical parts of the hardware, user interactions, and software components.
A runtime environment loads applications and has them run on a platform. All the resources necessary for running independently of the operating system are available on this platform. That’s why, for example, playing Flash videos is only possible with the right runtime environment - in this case the Adobe Flash Player. Within this environment, Flash videos can be played in the same quality and with the same functionalities regardless of which browser or operation system is being used.