The sparkplug can get fouled by a sticky or misadjusted carb, plugged air filter, or burning oil.  This is what a fouled plug looks like.  It makes the engine hard or impossible to start.  What happens is that the spark runs down the center insulator instead of jumping to the prong on the top - so it doesn't set off the fuel.

You may have to replace the sparkplug if you don't have the right equipment to clean it.

B&S says to NEVER sandblast a plug to clean it (but I don't know a better way).

This is an example of a bridged sparkplug.  A grain of dirt or carbon bridges the spark gap.  Instead of a spark, the ignition just runs through the bridge.  Just clean out the bridging trash.