Ubuntu Mate is an open-source Linux distribution and is regarded as an official “flavor” of Ubuntu by Canonical. The main difference between Ubuntu and Ubuntu Mate is that the mate version makes use of the MATE desktop environment.
MATE is a fork of the GNOME 2 desktop interface, whereas the main branch of Ubuntu has now moved on to the GNOME 3 interface.
There are several reasons you might want to use Ubuntu Mate for your Raspberry Pi instead of Raspbian. One of those reasons is that you gain access to the usually more modern packages available through Ubuntu.
Debian on which Raspbian is built off traditionally sticks with older more tested and bug-free versions of applications
Ubuntu Mate is currently only available for the Raspberry Pi 2, 3 and 3B+. So make sure you’re going to use one of these before starting. Support for the Raspberry Pi 4 is likely to come in the future.