Someone posted it on Hackernews. One of the comments said, roughly, that they didn't see the significance and could someone "explain it like I'm a Computer Science undergrad." This is my attempt to reply...
Um. Now I feel like I'm 106 instead of "just" 53.
OK, so, basically all modern mass-market OSes of any significance derive in some way from 2 historical minicomputer families... and both were from the same company.
Minicomputers are what came after mainframes, before microcomputers. A microcomputer is a computer whose processor is a microchip: a single integrated circuit containing the whole processor. Before the first one was invented in 1974 (IIRC), processors were made from discrete logic: lots of little silicon chips.
The main distinguishing feature of minicomputers from micros is that the early micros were single-user: one computer, one terminal, one user. No multitasking or anything.
Minicomputers appeared in the 1960s and peaked in the 1970s, and cost just tens to hundreds of thousands of dollars, while mainframes cost millions and were usually leased. So minicomputers could be afforded by a company department, not an entire corporation... meaning that they were shared, by dozens of people. So, unlike the early micros, minis had multiuser support, multitasking, basic security and so on.
The most significant minicomputer vendor was a company called DEC: Digital Equipment Corporation. DEC made multiple incompatible lines of minis, many called PDP-something -- some with 9-bit logic, some with 12-bit, 18-bit, or 36-bit logic.
One of its early big hits was the 12-bit PDP-8. It ran multiple incompatible OSes, but one was called OS-8. This OS is long gone but it was the origin of a command-line interface with commands such as DIR, TYPE, DEL, REN and so on. It also had a filesystem with 6-letter names (all in caps) with semi-standardised 3-letter extensions, such as README.TXT.
This OS and its shell later inspired Digital Research's CP/M OS, the first industry-standard OS for 8-bit micros. CP/M was going to be the OS for the IBM PC but IBM got a cheaper deal from Microsoft for what was essentially a clean-room re-implementation of CP/M, called MS-DOS.
So DEC's PDP-8 and OS-8 directly inspired the entire PC-compatible industry, the whole x86 computer industry.
Another DEC mini was the 18-bit PDP-7. Like almost all DEC minis, this too ran multiple OSes, both from DEC and others.
A 3rd-party OS hacked together as a skunkworks project on a disused spare PDP-7 at AT&T's research labs was UNIX.
More or less at the same time as the computer industry gradually standardised on the 8-bit byte, DEC also made 16-bit and 32-bit machines.
Among the 16-bit machines, the most commercially successful was the PDP-11. This is the machine that UNIX's creators first ported it to, and in the process, they rewrote it in a new language called C.
The PDP-11 was a huge success so DEC was under commercial pressure to make an improved successor model. It did this by extending the 16-bit PDP-11 instruction set to 32 bits. For this machine, the engineer behind the most successful PDP-11 OS, called RSX-11, led a small team that developed a new, pre-emptive multitasking, multiuser OS with virtual memory, called VMS.
(When it gained a POSIX-compliant mode and TCP/IP, it was renamed from VAX/VMS to OpenVMS.)
OpenVMS is still around: it was ported to DEC's Alpha, the first 64-bit RISC chip, and later to the Intel Itanium. Now it has been spun out from HP and is being ported to x86-64.
But the VMS project leader, Dave Cutler, and his team, were headhunted from DEC by Microsoft.
At this time, IBM and Microsoft had very acrimoniously fallen out over the failed OS/2 project. IBM kept the x86-32 version OS/2 for the 386, which it completed and sold as OS/2 2 (and later 2.1, 3, 4 and 4.5. It is still on sale today under the name Blue Lion from Arca Noae.)
At Microsoft, Cutler and his team got given the very incomplete OS/2 version 3, a planned CPU-independent portable version. Cutler et al finished this, porting it to the new Intel RISC chip, the i860. This was codenamed the "N-Ten". The resultant OS was initially called OS/2 NT, later renamed – due to the success of Windows 3 – as Windows NT. Its design owes as much to DEC VMS as it does to OS/2.
Today, Windows NT is the basis of Windows 10 and 11.
So the PDP-7, PDP-8 and PDP-11 directly influenced the development of CP/M, MS-DOS, OS/2, Windows 1 through to Windows ME.
A different line of PDPs directly led to UNIX and C.
Meanwhile, the PDP-11's 32-bit successor directly influenced the design of Windows NT.
When micros grew up and got to be 32-bit computers themselves, and vendors needed multitasking OSes with multiuser security, they turned back to 1970s mini OSes.
This project is a FOSS re-implementation of the VAX CPU on an FPGA. It is at least the 3rd such project but the earlier ones were not FOSS and have been lost.