The multi-window feature is a feature provided by Samsung on devices including (but not limited to):

• Galaxy Note 2
• Galaxy Note 3
• Galaxy Note 4
• Galaxy Note Edge
• Galaxy Note 10.1
• Galaxy Note 10.1 2014 Edition
• Galaxy Note 8.0
• Galaxy Note Pro 12.2

It allows you to run 2 or more applications at the same time. To use this feature, make sure you have Squid 0.4 or later and the latest update from Samsung for your device.