From the video description:
SMS2 (short for Small Multitasking System 2) is a multitasking operating system developed by Tony Tebby (QJUMP Ltd.) in the early 1990s and published by a company called FURST Ltd. in 1993.
Tony Tebby is probably best known for having initiated the development of the operating system for the Sinclair ZX83 project in 1982/1983.
The most advanced member of QDOS compatible operating systems is SMSQ/E which is still being developed.
via Carsten Strottman