The headline unemployment-rate is U-3 and counts only people actively searching for work. U-6 adds those who have given up looking and those working part time for economic reasons, so it is typically several points higher.
The gap between U-6 and U-3 is the informative part. A widening gap while the headline rate is flat means slack is building beneath the surface, usually through cut hours before outright layoffs, which is the classic early sign of a cooling labour market.
Example: U-3 is 4.1% and U-6 is 7.8%, a 3.7 point gap against a long-run average near 3.4. Six months later U-3 is 4.2% but U-6 is 8.6%, so the deterioration is four times larger than the headline suggests.
Related: unemployment-rate, labour-force-participation, household-survey, jolts, sahm-rule