The <10 cases per 100,000 residents. The population of the county is 171,000, so less than 17 active cases in the county in order to move to the next phase. And since an "active case" is considered any positive test result in the previous 14 days, a person with multiple positive tests is one case for each test. There are counties in this state that have had single digit cases for the entire pandemic that are considered "Areas of High COVID Activity."
It would have been nice if the had published the metrics for each of the different phases (I couldn't find them with a search), but my read on the <10 cases per 100,000 residents was that this was the target for resuming full in-person instruction, if they're willing to do hybrid instruction with their current load of 120ish cases per 100,000.
Our governor, on the other hand, is indicating no high school hybrids until <25 cases per 100,000 resident. We haven't been there since mid-March, and as a few more things here and there open up, this doesn't seem like it's going to happen anytime soon.