OS in coding means Operating System.
In computer coding an operating system is a software program that is required to manage and operate a computing device like smartphones, tablets, computers, supercomputers, web servers, cars, network towers, smartwatches, etc.
It is the operating system that eliminates the need to know coding language to interact with computing devices.
For example if you use Windows 10 on your computer than that is your computers operating system.
If you use Linux then Linux is your operating system.
Before regular easy to use operating systems were developed people had to use DOS on their computers which was more complicated.
The DOS was Disk Operating System.
Linux, macOS, and Windows are highly preferred operating systems for web developers.
Although, Windows has an additional advantage as it allows to work simultaneously with Windows and Linux.
Using these two Operating Systems allows web developers to use the necessary apps including Node JS, Ubuntu, and GIT.