Segwit is a code change that fixes transaction malleability, improves security, and sets the foundation for future scaling and development.
(more at bitcoincore.org)
How to Measure Support For Change
Miners can start signalling (voting) for SegWit after the first difficulty retarget after November 15th.
Mining difficulty is retargeted to maintain 10 min blocks:
At each retarget, support for segwit is measured based on previous signalling in previous 2016 blocks.
How Do Miners Signal Support?
No support/status quo:
If 95% of blocks mined signal support, the bitcoin 0.13.1 will be activated at next retarget.