A leverage point is any place where a relatively small amount of work you do can generate a big result. Typical examples include preventing a problem in the first place, rather than addressing it after it's exploded, or gaining someone as a true ally in your cause, versus paying or manipulating them into doing one thing for you.

Donella Meadows wrote a paper about leverage points which I find very helpful in thinking about what the most powerful leverage points might be. http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf