Between the date choosen of August 1st and November 15th, UASF nodes will consider all blocks invalid that do not explicitly signal support for SegWit, thus creating a fork. Within that fork, support for SegWit will be 100% which in turn triggers the activation threshold of SegWit (95%).
So, If the UASF is activated successfully then that's going to happen:
1) nodes running UASF will reject blocks which do not signal for segwit.
2) miners running UASF will reject blocks which do not signal for segwit.
All this is an attempt to force the activation of SegWit which is currently (~30%) unable to reach the required activation threshold (95%).
Why was the date of August 1, 2017 chosen?"Because BIP9 is time based, BIP148 needs to account for the possibility for some of the hash power to exit (eg. to mine another fork) which would make block intervals longer. The August 1st date allows for the economic majority to successfully activate SegWit. Theoretically, if the hashpower drops by up to 85%, it might take up to 13 weeks to complete an activation period. In this scenario, SegWit will still activate for all BIP148 compliant nodes."Source:
http://www.uasf.co/#why-was-the-date-of-august-1-2017-chosen