See: Okun's Law.
The Okun Gap is the opportunity costs of an inefficiently running economy, i.e. what could have been. The Okun Gap is the potential GDP minus the actual GDP, or potential output minus actual output.
When actual output is more than potential output—which sounds counterintuitive, but hang on—it’s called an “inflationary gap” (because of inflation, which is a nominal increase, rather than a real increase in value). When potential output is greater than actual output (i.e. we could be doing better), it’s called a “recessionary gap.”
So...that all makes sense, but how do we know what our “potential output” coulda-woulda-shoulda been?
Enter: the Okun Gap.
The Okun Gap is named after economist Arthur Melvin Okun, who saw all those unemployed people and said "what a waste of unused manpower for the economy!" (We’re paraphrasing here). The Okun Gap, whether inflationary or recessionary, is directly related to the unemployment rate, reflecting unused labor.