OpenWrt is an open source project for embedded operating systems based on Linux, primarily used on embedded devices to route network traffic. The main components are Linux, util-linux, musl, and BusyBox
If your router is been sitting collecting dust or you have bricked it by some reason , you can put it back to life where you can unbrick it the official way your manufacturer does , and then after that you can flash OpenWRT to it , it can be turned from old router that collects dust , to a powerful router that collects dust!
One of the features that it gives OpenWRT to be the choice is that the many features it provides to your router , unlike how it was , and the firewall that it can add to it
You can check for the supported devices in this link , and each device has it's own installing method - https://openwrt.org/supported_devices
DO NOT BUY DEVICES WITH 4MB FLASH / 32MB RAM if you intend to flash an up-to-date and secure OpenWrt version (18.06 or later) onto it!