The horizontal number is your chainring and the vertical one is your cog. A skid patch is the spot on your rear tire that will be touching the pavement when you skid. Because the rear wheel turns when the cranks turn, the wheel stops spinning when you apply back pressure on the pedals to brake. As the cranks go around, the wheel goes around in a ratio determined by your gearing. Certain ratios have the same patch of tire hitting the ground when the cranks are in the 9 o'clock/3 o'clock positions; your braking position. After so many skids, you'll wear a tire out so the more skid patches you have, the longer the tire will last.