In short, this will expand the range of frequencies reproduced through hands-free devices. The standard in use on the Android OS side dates from HDP v1.6 and is known in the telephony industry as Wideband. It is specified to cover frequencies from 50Hz to 7KHz and is also commercially termed as HD Voice. Delivery happens through a special, HFP-dedicated version of the SBC codec called mSBC.
In making the step from HFP v1.8 to v1.9 — which is still in the drafting stage — Android will upgrade to Super Wideband (50Hz-14kHz) and use the relatively new
LC3 codec to make that happen. That's just short of Fullband reproduction (20Hz-20kHz), which most consumer Bluetooth audio gear is capable of.