An excellent essay from Ink & Switch introduces end-user programming as follows:

Today’s software apps are like appliances: we can only use the capabilities exactly as programmed by the developer. What if we, and all computer users, could reach in and modify our favorite apps? Or even create new apps on the fly according to our needs in the moment?

This is end-user programming, a vision for empowered computing pursued by bright-eyed computer science visionaries. Its rich history reaches back to the 1960s with programming environments like Smalltalk and Logo. Notable successes since then include Unix, the spreadsheet, Hypercard, and HTML. And today, newcomers like Zapier, Coda, and Siri Shortcuts are trying their own approaches to automation and dynamic modeling.

SilverBullet is working towards supporting more end-user programming features. Some of them currently available include: