The way I like to think about is that in the long run, earnings will revert to their mean level due to competitive forces.
So when you are analyzing a company with heavy use of accruals, for example deferring large amounts of revenue to manage a steady upward trend in profitability going forward, the current earnings level is relatively low compared to a similar company that correctly recognizes revenue (all else being equal).
Because the low earnings level is not representative of the future earnings capability of the company, it is said to be of lower quality or less persistent - because the deferred revenue balance will eventually be recognized as revenue, the low revenue will not “persist” in the future.