The smartphone market, and the consumer market in general, is designed in such a way that things become obsolete very quickly. As such, in mobile terms, the Galaxy S5 is something of a relic. But with a full-HD display, excellent camera with 4K video and waterproof housing, it's little wonder that many S5 owners have felt compelled to hang on to the device. If you're still rocking the Galaxy S5 carried by Big Red, a sizable care package is on its way through cyberspace.
Software version MMB29M.G900VVRU2DPD1 brings users up to Android 6.0.1 Marshmallow. This means you get all of the features of Android 6.0 including Doze and Google Now on Tap, along with the smaller additions added with the 6.0.1 bump.
The update should reach you very soon if it hasn't already. As ever, you can push things along by opening the Settings app on your S5, navigating down to About device, and checking manually for your update.
The full changelog, meanwhile, can be seen at the source.