Ya know, why do companies make you read three paragraphs before even mentioning anything about what the thing actually is?
tldr: Microsoft is building a Linux based system to run network switches. This is part of a trend in the industry that has been moving more network intelligence into abstracted software rather than proprietary hardware. Background: https://en.m.wikipedia.org/wiki/Software-defined_networking
Interesting discussion in comments below the article about if this will open sourced at some point in future. If I understand it correctly MS is not under any obligation of open sourcing this which pisses me off considering how Bill Gates was against the whole open source concept(wrote many letters supporting his argument).
Bill Gates has not been running the company for many years. Whether it is open sourced or not has nothing to do with him. I expect it will depend on how much value they see open sourcing will be to the community will be an overriding factor.
OpenWrt is meant for single router/switch installations in the home or office.
These switch operating systems are for building large fabrics for out of 48-port 10g/40g top-of-rack switches (soon to perhaps be 25g/100g switches?). They support automated configuration management, large BGP configurations (often through Quagga), probably some layer 2 magic, and probably some hardware acceleration for network virtualization on the later Broadcom chips.
tldr: Microsoft is building a Linux based system to run network switches. This is part of a trend in the industry that has been moving more network intelligence into abstracted software rather than proprietary hardware. Background: https://en.m.wikipedia.org/wiki/Software-defined_networking