UASF stands for User Activated Soft Fork. It’s a mechanism where the activation time of a soft fork occurs on a specified date enforced by full nodes, a concept sometimes referred to as the economic majority. A UASF requires a lot of industry support and coordination, which is good practice for eventual hard forks which requires even more industry coordination. In the past, a UASF was successfully carried out to activate the P2SH soft fork (BIP16). The UASF concept was combined with SegWit activation in the BIP148 proposal which can be found on this GitHub:
What is BIP148
BIP148 is a UASF that is designed to cause the existing SegWit MASF deployment to cause activation in all existing SegWit capable node software (which currently is 80% of the network nodes). How does BIP148 Work? From August 1st, 2017, miners are required to signal readiness for SegWit by creating blocks with the version bit 1. This will cause all SegWit ready nodes, which make up over 80% of the network, to activate and begin enforcement. Link for reference: luke.dashjr.org/programs/bitcoin/files/charts/segwit.html. Miners must also check blocks prior to their own and ensure that they also signal for SegWit, and only build on those blocks.