BIP is Bitcoin Improvement Proposal. each of them are different proposals to improve bitcoin in one way or another. BIP148 is User Activated Soft Fork or UASF for short.
https://github.com/bitcoin/bips/blob/master/bip-0148.mediawikiit was supposed to "force" miners to accept SegWit by rejecting any block that is not signalling for SegWit by the time specified in the BIP.
but now that time is past and also since 100% of the blocks are signalling for SegWit nothing happened. in other words there was nothing to reject.