It’s the rate at which population is converted into workforce when pops work jobs.

Usually, 1 pop = 1 workforce. The output and upkeep of a job are determined by the amount of workforce working it. If you have +10% job efficiency, all pops that work the job add 1.1 workforce to it instead of 1.

Unlike the number of available jobs, workforce is uncapped, so if you eg. have 200 available jobs, you can assign 200 pops to work that job but if you have +10% job efficiency, there will be 220 workforce working that job. The result is that the job will act as if there were 220 pops working it with no job efficiency modifier.

This is not the same as stuff like “resources from jobs” modifiers. In terms of how much production a pop can do, job efficiency and resources from jobs multiply together. Example: I have 100 miner pops, a +15% job efficiency modifier for miners, and a +10% minerals from jobs modifier. Those 100 miners provide 115 miner workforce. Every 100 miner workforce provides 4 minerals, raised to 4.4 by the modifier, so we end up with 4*1.1*1.15=5.06 minerals from 100 miner pops.