I believe that a layman's explanation is that BIP91 facilitates (or forces) the locking in and activation of segwit.  The beginning of the forcing of the signaling of segwit (through BIP141) will start after the 350 block grace period that already started.. which is really in a few days, but will cause the 95% locking in of segwit, which will thereafter cause segwit to activate in the terms of BIP141 , which locking in about mid August .

BIP91 also causes some attention to a possible 2mb blocksize limit increase through a hardfork - however, the language of BIP91 does not seem to mandate those further steps regarding 2mb increase or the hardfork (as mentioned by -ck above).

So the more important thing about BIP91 seems to be providing a vehicle or a channel to activate segwit - which also nullifies the activation of BIP148 (User Activated Soft Fork) that would have gone into effect on 8/1.. but is now unnecessary because of this whole matter of seg wit already going to be on the path of activation, which is what BIP148 would have attempted to cause.

Can anyone say more than that regarding what BIP91 actually causes?  Maybe causes a sense of comfort in the markets to have some kinds of resolution and direction of the segwit portion of the bitcoin fundamentals matter.


[woops I edited some of the above text, and then I saw below that -ck had responded & cited the earlier version]