What the four quadrants mean
The Eisenhower Matrix sorts tasks along two axes: urgency and importance. The result is four quadrants. Quadrant one (urgent and important) is for crises and hard deadlines that you do now. Quadrant two (important, not urgent) holds planning, prevention, and relationship-building, the work that produces long-term results. Quadrant three (urgent, not important) covers interruptions and most meetings, which are good candidates to delegate. Quadrant four (neither) is distraction to drop.
The method is attributed to a remark by Dwight D. Eisenhower that the urgent is rarely important and the important rarely urgent. Stephen Covey popularized the grid in The 7 Habits of Highly Effective People.
Why quadrant two is the leverage point
Most reactive workers live in quadrants one and three. The practical insight of the matrix is that deliberately investing in quadrant two reduces the volume of future quadrant-one fires, because prevention and planning stop crises before they start.