It's the advancement of a base at it's core. A player who goes 1-1 with a single has an OPS of 1.000, while the same 1-1 with a homer is 4.000. A team with a 1.000 OPS should score every inning at worst. Lead-off homer, 4 bases, 1 AB and three outs. 4 bases, 4 AB, 3 outs.