Any procedure or action that solves a problem, simplifies a task or reduces frustration in one's everyday life. For example: If you are a disorganized person, I can tell you about a few life hacks that will help you!

Here are some great examples: http://www.epicdash.com/the-46-most-brilliant-life-hacks-every-human-being-needs-to-make-life-easier/

It's not standard English. It comes from computer jargon of the 1960s. A "hack" can mean a clever, surprising, unexpected way to solve a difficult computer programming problem. It can also mean a clever way to break into a computer system you're not supposed to get into. Since these are "computer hacks," by extension, a "life hack" is a clever, surprising, unexpected way to solve a problem in everyday life.