What is a "Distro"?
Distro is an abbreviation of "distribution," and typically refers to an operating system distributed (i.e. that can be installed) based on Linux, or, in some cases, on BSD. For example, Ubuntu, Linux Mint, Debian, Fedora, Gentoo, OpenSUSE, Arch Linux, Alpine Linux, Manjaro, Pop!_OS, SteamOS, and Kali Linux are Linux distros, while FreeBSD, OpenBSD, GhostBSD, and NetBSD are BSD distros. BSD systems are less popular than Linux, specially for desktop, so "distro" normally refers to a Linux distro, not to a BSD distro.
Although we don't call different versions of Windows "distros," in some cases the phrase "a Windows distro" may be used as a joke by Linux enthusiasts.