Slack is absence of binding constraints on behavior. The term is usually capitalized to distinguish it from the ordinary English meaning. Not to be confused with the communication app by the same name.

From the post which introduced this usage, Slack:

Poor is the person without Slack. Lack of Slack compounds and traps.
Slack means margin for error. You can relax.
Slack allows pursuing opportunities. You can explore. You can trade.
Slack prevents desperation. You can avoid bad trades and wait for better spots. You can be efficient.
Slack permits planning for the long term. You can invest.
Slack enables doing things for your own amusement. You can play games. You can have fun.
Slack enables doing the right thing. Stand by your friends. Reward the worthy. Punish the wicked. You can have a code.
Slack presents things as they are without concern for how things look or what others think. You can be honest.
You can do some of these things, and choose not to do others. Because you don’t have to.
Only with slack can one be a righteous dude.
Slack is life.

Related Sequence: Slack and the Sabbath