Some aspects are specific to the U.S. regulations, but I will try to simplify the concept of the hazard quotient.
Often, concentrations in the environment will be measured and compared with known data on thresholds where there is an effect, a toxicity or an impact on health, among other things. If you fall below the thresholds, you have a lower hazard quotient; if you are above the thresholds, you have a problem.
In the case of PFAS, the problem is that these comparisons tell us that we are all overexposed. In our blood, we all have a PFAS concentration above thresholds, and they are starting to affect the immune system and cholesterol, for example, and increase the risk of cancer.