Segwit2x was a working group tasked to design, implement, build, test, and deploy the software required to scale the current Bitcoin network, as described in the agreement reached on May 21, 2017, in New York between significant Bitcoin companies. That agreement, usually abbreviated as NYA, is another landmark in the Scaling Debate inside Bitcoin.
Segwit2x supported Segregated Witness (or SegWit; mainly, a transaction format optimization) which was activated in Bitcoin on August 2017, but it also planned to increase Bitcoin's block size limit from 1MB to 2MB. Proponents of Segwit2x believed the increase in block size will allow more Transactions Per Second (TPS), which is necessary for Bitcoin to be usable for business.
The block size increase would be done through a hard-fork, scheduled to take place at block height 494,784, which would be around November 16.
Segwit2x's planned hard-fork of Bitcoin was not supported by the development team of Bitcoin Core, which is the "reference" Bitcoin wallet/node software app.