This is a really good piece of news.
They're moving from a custom license which restricted commercial use to a BSD 3-Clause one for most of the project. The main thing this will do is allow the original copyright holders of the games to make use of the code created by the MAME developers when releasing their own retro packages.
Originally this was prohibited because the MAME developers wanted to protect themselves from being seen as helping facilitate the creation of unofficial game boards running the emulation software instead of using the original components. Even bare boards without ROM files included could have brought about unwanted legal action against the developers.
The news here is two-fold. The BSD-3 movement comes mostly from the desire of the developers of the sister project MESS who have been seen their software dismissed by museums and educational institutes because of the custom license despite having the highest quality emulation for a large number of obscure hardwares.
The move to change MAME to BSD-3 comes at the same time as the two projects are being combined into a single new MAME entity that does everything from arcades, handheld games, popular 8-bit, 16-bit and even some 32-bit home systems to other machines that were only ever used in business. This is a huge step in trying to make MAME a more useful application for a wider audience and a very generous move by the developers. It has taken almost 20 years but MAME is finally growing up and making a case to be considered as a serious piece of software.