Mojave (pronounced "mo-ha-vee") is the name of macOS 10.14, the fifteenth version of macOS (previously OS X). It was released on September 24th, 2018 and follows macOS 10.13, also known as High Sierra.
macOS Mojave is a minor update to macOS but includes a few notable new features. One of the most significant is Dark Mode, which makes the menu bar and window backgrounds dark. It also changes the light grey and white backgrounds of supported applications to dark grey with white text. Dark mode was inspired by the dark interface of software development IDEs used by programmers and is intended to reduce eye strain.
Other UI improvements in Mojave include "time-shifting" desktops that change color during the day, a new Gallery View in the Finder, and Stacks, which automatically groups similar file types together. Mojave also includes new document editing capabilities at the operating system level. For example, the updated Quick Look feature allows you to mark up a PDF or edit an image without even opening it with an application. You can also edit and share screenshots immediately after taking them. The screen capture feature in Mojave is the first to support screen recording.
macOS Mojave is the first version of macOS to include support for iOS apps. While only a handful of Apple's own iOS apps were supported by Mojave at the time of launch, Apple plans to allow third party developers to build crossplatform apps that run on both iOS and macOS in the future.