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You place one in between the rod bearing and crank pin journal and tighten to specifications. When removed, simple measurements on the markings on the squashed plastigauge will very precisely determine the clearance, taper, and out-of-round specifications for your bearing. It is also possible to obtain most of this information by using slips of paper or foil of known thickness but this will not be as accurate or convenient.
Fortunately, the metal ridge is rarely a problem on lawn mower engines and the carbon ridge can be removed with a simple homemade tool which is just a soft metal (i.e., aluminum or brass) piece with a straight edge or inside right angle. You probably will not need an expensive commercial ridge reamer tool. You will not need one at all unless you will be removing the piston.