Cardinal altitude refers to odd or even thousand-foot altitudes or flight levels (i.e., 5000, 6000, 7000, FL 250, FL 260, etc.).
Cardinal altitudes are of prime importance for use in aircraft separation, which is a direct issue of safety and therefore a top priority. Instead of air traffic controllers enforcing separation minimums for each individual aircraft relative to where they happen to be, controllers assign aircraft to fly at the specific increments of altitude or flight levels known as cardinal altitudes.
For all flights between 3000 ft above the ground and FL 410 (41,000 ft. MSL), aircraft are required to fly cardinal altitudes or flight levels at increments of 2,000 ft based on which direction they are flying. For example, all aircraft with a magnetic heading of 0 to 179 degrees fly at an "odd plus 2,000 ft" (i.e., 3000, 5000, FL 290, FL 310, etc.) and all aircraft with a heading of 180 to 359 degrees must fly "even plus 2,000ft." (i.e., 4000, 6000, FL 260, FL 280, etc.)