SegWit (Segregated Witness)

The following article is an excellent source for explaining SegWit or Segregated Witness:

https://cointelegraph.com/explained/segwit-explained

Here is a summary taken directly from the article linked above:

Segregated Witness, often abbreviated to SegWit, is a proposed update to the Bitcoin software, designed to fix a range of serious issues. SegWit increases the Bitcoin’s block size limit and allows the implementation of the second-layer solutions for further improvement. [From currently 1MB to between 2 and 4 MB depending on the network]. In order to avoid a fork, or new currency, SegWit is set to activate as soon as 95 percent of the network measured in hash power signals support for it [Current support level at the time of this articles posted was 30 - 35 %]. The key points against SegWit can be roughly divided into three groups: technical, political and ideological. [All have some good points - read the article for more.]

Current segwit support level can be checked here:
https://coin.dance/blocks

This information all came from the above article written by Andrew Marshall of Coin Telegraph. My own words or paraphrases of Mr. Marshall's content are in brackets.