Sealcoat is a liquid coating, usually an asphalt emulsion or refined coal tar mixed with water and sand, applied in thin coats over asphalt to protect it from oxidation, water and fuel spills.
It's maintenance, not repair. Sealcoat adds no strength, doesn't fill ruts and doesn't fix cracks, which is why crack sealing goes first and gets priced separately. On a sound lot it slows the graying and raveling that lead to bigger work later. How often a lot needs it depends on traffic and climate, and every few years is a common rule of thumb.
Check the law before you price coal tar. Minnesota, Washington, New York, Maryland and Washington, D.C. ban coal-tar sealers, and so do a long list of cities and counties.
Material is figured from the manufacturer's application rate. Spec sheets usually give it in gallons of mixed material per square yard per coat. Divide 9 by that number to get square feet per gallon, so 0.12 gallons per square yard is 75 square feet per gallon. Rough, oxidized pavement takes more than a lot sealed a couple of years ago, and most lots get restriped on the same job.