From the OpenWrt.org website: OpenWrt is a highly extensible GNU/Linux distribution for embedded devices (typically wireless routers). Unlike many other distributions for these routers, OpenWrt is built from the ground up to be a full-featured, easily modifiable operating system for your router. In practice, this means that you can have all the features you need with none of the bloat, powered by a Linux kernel that's more recent than most other distributions.
Instead of trying to create a single, static firmware, OpenWrt provides a fully writable filesystem with optional package management. This frees you from the restrictions of the application selection and configuration provided by the vendor and allows you to use packages to customize an embedded device to suit any application. For developers, OpenWrt provides a framework to build an application without having to create a complete firmware image and distribution around it. For users, this means the freedom of full customization, allowing the use of an embedded device in ways the vendor never envisioned.
OpenWrt 18.06.4 incorporates a number of bug fixes in the network and system userland, as well as updates to the kernel and base packages.
Some selected highlights of the new service release are:
Linux kernel updated to versions 4.9.184/4.14.131 (from 4.9.152/4.14.95 in v18.06.2)
SACK security fixes for the Linux kernel
WPA3 security fixes in hostapd
Further security fixes for Curl and the Linux kernel
MT76 wireless driver updates
Various network and system service fixes
All the information about thios release can be found here: https://openwrt.org/releases/18.06/notes-18.06.4