Okun's law is the bridge between the goods market and the labour market. It is what lets a forecaster translate a growth forecast into an unemployment forecast, and it underpins the arithmetic of how much of a slowdown a soft-landing requires.
It is a regularity rather than a law, and it breaks when firms hoard labour through a downturn or when productivity swings sharply. Both happened after 2020, which is part of why unemployment behaved so differently from what growth alone implied.
Example: to close a plus 1.2% output-gap, growth must run below potential by 1.2 points for a year. With a coefficient of 2, that implies unemployment rises by 1.2 / 2 = 0.6 percentage points, say from 3.8% to 4.4%.
Related: output-gap, nairu, unemployment-rate, potential-gdp, soft-landing