There may have been problems with the tile that left gaps at the bottom of the toilet. If so, he may have used shims to make it level and the sealant is covering the shims. You can use sealants or grout around the bottom of the bowl and there are color choices available. I recommend not sealing the rear portion of the bowl so if there is a leak, you will know it before it rots the subfloor out.
if you have never made a mistake, you probably haven't done much.