BIP 91 is the first step towards implementing the SegWit2x scaling proposal, which is set to rescue bitcoin from the pains of an overwhelmed and congested network and restore its competitiveness as a global internet money system.
As of 11am HKT, 20 July, over 75% of blocks mined today supported BIP 91. For SegWit to be activated, 80% or 269 blocks over a 336-block period need to signal BIP 91.
If this happens before 31 July, then BIP 91 will overrule BIP 148, also known as the User Activated Soft Fork (UASF) — a proposal with great risk of causing a split in the bitcoin network, as it rejects any block from miners that do not signal SegWit readiness on 1 August.
If you’re still confused by all the BIPs and HFs, check out the visual guide to scaling outcomes by Coindesk below:
However, the real issue is not what happens by 1 August, but what is going to happen in November with the proposed increase in the block size as part of SegWit2x, which requires a hard fork (and potential split) since not all proponents of SegWit agree that a hard fork is necessary.
Reference: https://blog.gatecoin.com/blockchain-asset-weekly-20-july-2017-75f820c3709b