BIP148 is Bitcoin Improvement Proposal number 148.
It is essentially a User Activated Soft Fork (UASF) which is being driver by users of bitcoin to increase scalability .
The UASF groups are trying to force this version of SEGWIT (Segregated Witness) on the miners who stand to loose out in payments as BIP148 will reduce their fees for each transaction.
This version of segwit will activate on Aug 1st 2017 at the latest.
The danger is that if you have BTC in an exchange or location that does not support segwit version BIP148 your BTC may not exist on other exchanges when you send it or try to buy something with it.
So, in order to play safe with your Bitcoin, sell them for dollars or Euro or get them into your own possession in a paper wallet or a wallet on your PC or somewhere where you are the holder of the private keys.
Hold until after the BIP148 dust settles and we see if BIP148 has been adopted by the miners or not.