The Security Update 2016-003 includes additional kernel issue fixes that have the potential to cause Macs running the operating system to freeze up and become unresponsive. According to Apple "Security Update 2016-003 is recommended for all users and improves the security of OS X".
The Security Update 2016-003 is now rolling out as a free update to all OS X El Capitan 10.11.6 users, or can be downloaded through the link down below.
Apple