Translating and running code
Source code is written for people to read and edit. A compiler translates it into machine instructions or another form a computer can use, often catching errors before the program runs.
The runtime is the software and environment a program relies on while it runs. It may manage memory and provide built-in features or access to other resources. A runtime error occurs while the program runs, rather than during compilation.
Compilation might happen on a developer’s computer or a CI runner, while the program runs on a server, in a browser, or on a phone. Build and runtime problems may therefore appear on different machines.
The boundary is not always neat
Some programs are translated before they run, some while they run, and some use both approaches. Product teams may encounter the distinction when a build fails before release or an app fails while people use it.