What is a runway incursion and why is it significant?

According to the FAA, a runway incursion is "Any occurrence at an aerodrome involving the incorrect presence of an aircraft, vehicle or person on the protected area of a surface designated for the landing and take off of aircraft." It's "significant" because people get killed that way. Runway incursion results in collosion of aircraft on the ground when they are fully loaded with fuel and people. Runway incursion is when something enters an active runway.

Runway excursion is when a aircraft departs a runway in error.