what is emission? My check engine light came on and had it checked and they said it was emission

This means that you car is producing air-pollution and there are regulations on how much pollution per car can be produced. Hydrocarbons, nitrogen oxides, carbon monoxide, and carbon dioxide are produced through the tailpipe as well as emissions that are produced from evaporation of fuel. You can read up on these emissions at: http://en.wikipedia.org/wiki/Automobile_emissions_control

Do you know what the code was? If you do I can give you a better idea of what the problem is.