For exact technical explanations the Wikipedia page on Micro 4/3
is a good starting point, but what it basically boils down to is that MFT cameras have a smaller sensor and no mirror in between which has an influence on the flange focal distance with lenses. Still too technical? Just like on many other advanced cameras you can change lenses on MFT cameras. The big difference is that you can put pretty much ANY lens on your MFT camera with an appropriate adapter. This is a huge advantage because it means you can use lenses that you already own, or buy high quality vintage lenses for a very low price. Just to give you an example; some time ago I bought a suitcase filled with lenses on a flea market for $50,-. These were old lenses from back in the 80’s: wide angle lenses, zoom lenses, portrait lenses, all very high quality glass that is as good today as it was 30 years ago. Because the lenses were all made for the old Olympus OM camera I could order an OM to MFT adapter on eBay for around $20,- and have a professional range of lenses for a total of $70,-.