A Linux distro, or distribution to use its full name, is an operating system built around the Linux kernel. Unlike Windows or Mac OS, a Linux operating system consists of many open-source components produced and managed by different organizations.
While you could obtain and assemble all of these programs yourself, it's time-consuming and hard to get everything set up. That's where distros come in. They do all the hard work, deciding which open-source software to use and presenting it in a format that's ready for you to use with minimal configuration.