Definition - What does BIP 148 mean?
BIP 148 is a user-activated soft fork (UASF) in bitcoin that enforces the use of the Segregated Witness or SegWit protocol, which is a change that deals with a bitcoin block size problem by separating the digital signature from the original section of the block. As a soft fork, BIP 148 does not inherently split the network. BIP 148 requires bitcoin miners to signal for SegWit, which means they show that they are ready to use blocks with the SegWit protocol.
Techopedia explains BIP 148
Over the summer of 2017, BIP 148 was proposed by members of the bitcoin community. On implementation, bitcoin miners rallied and signaled support for SegWit, preempting further action. However, other events like a miner-activated fork are on the horizon for bitcoin. Essentially, forks happen when there is not node consensus on bitcoin, when not everyone agrees to do the same thing. Because bitcoin is a non-regulated, decentralized system, there are any number of disagreements that result in forks. Hard forks can split the network. For instance, the emergence of Bitcoin Cash is an example of a hard fork that resulted in two separate bitcoin cryptocurrencies.
Understanding BIP 148 and related efforts means understanding how bitcoin miners and users work. Miners look for available blocks, and have their own incentives. Users may propose a rule that miners do not want to implement. The rest is largely semantic juggling related to trying to build consensus and control the inherent chaos of an unregulated cryptocurrency system.