Short answers, pulled from the story.
Segregated Witness, formally known as Segregated Witness (Consensus layer), is an implemented soft fork change in the transaction format of bitcoin. This upgrade carries the identifier BIP141 and serves to prevent nonintentional bitcoin transaction malleability.
Segregated Witness activated on the 24th of August 2017 at block height 481,824. The activation window for the SegWit upgrade started at Midnight the 15th of November 2016 UTC and ran until Midnight the 15th of November 2017 UTC across the target adjustment period.
Segregated witnesses solve this issue by serializing signatures separately from the rest of the transaction data. This separation ensures the transaction ID remains stable and immune to external tampering before confirmation.
Developers introduced a strict block size limit of 1 MB to protect the network after malicious actors exploited the lack of limits by creating fake block data that was very long. These attacks functioned as denial-of-service attempts designed to slow down the entire system.
As of February 2018 SegWit transactions exceed 30% of total activity on the main network. In the first week of October the proportion of network transactions using SegWit rose from 7% to 10%.