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What is SegWit?

Segregated Witness, often abbreviated to SegWit, is a proposed update to the Bitcoin software, designed to fix a range of serious issues.
Without getting technical, it makes transactions more efficient and would make further upgrades much easier and possible

What are the barriers to adoption? Does it require a fork in the blockchain?

It is my understanding that it would require a soft fork

"Once miners representing 95 percent of hash power (1,916 blocks) within a single difficulty period (2,016 blocks/about two weeks) include the right version bit, the soft fork is locked in. One difficulty period later, the soft fork is activated, meaning the remaining 5 percent of miners have about two weeks to upgrade."

Ah, fascinating...