It compares a target benefit — a share of monthly income — against what would keep arriving during a disability (existing group or individual benefits, a spouse's income, investment income) and against essential monthly expenses. The result is the additional monthly coverage needed and the shortfall before any new coverage.
For producers, the essential-expenses figure is gold: it tells you the minimum benefit that keeps the household afloat, which frames the conversation about benefit amount, elimination period and own-occupation definitions. See the disability insurance calculator.