Flesch reading ease
Scores run from 0 to about 100, and higher is easier. 60 to 70 is plain English that most adults read comfortably. Most popular newspapers aim for 60 to 70; academic papers often score below 30.
Grade level
Flesch-Kincaid, Gunning fog, SMOG, Coleman-Liau and the Automated Readability Index each estimate the US school grade needed to understand the text. They use slightly different signals (syllables, long words, letters per word), so the average is usually the fairest single number.
How to make text easier
Shorten long sentences, swap long words for short ones where the meaning holds, and break up big paragraphs. Readability formulas measure length, not meaning, so read your text aloud as a final check.