To scarify means to scratch or etch the surface of a seed coat before sowing it. Some seed coats are very tough and benefit from a little help before they are planted. Sweet Peas, or in my case Morning Glory, need a little roughing up before they hit the dirt.
I use a small file and run a few scratches across the coat, just deep enough to knick the shell but not so deep as to damage the seed itself. A good soaking in water for a few hours helps penetrate the seed coat and the seeds are ready to be sown.