What Does Distro Mean?
Distro is IT vernacular for a Linux operating system (OS). It is a shortened version of the term distribution.
The singular and the plural (distro vs distros) are often used synonymously.
Techopedia Explains Distro
In the Linux world, there are hundreds of different flavors of distro. Examples include Debian, Ubuntu and Red Hat (among many others). When referencing a Linux OS, administrators usually ask questions like: Which distro does the application sit on top of? or Which distro are you using?
Technically, a distribution is any deployment of software and isn't specifically Linux. Usually in this case you'll hear it referred to as a "distribution of software." Despite this, the term "distro" is nearly always used in a Linux context. So, while a distribution of software is a generic term for any bundle of software, distro is nearly always a flavor of Linux.