What keyword density tells you
Keyword density is how often a word or phrase appears, as a share of all words. Modern search engines do not reward a target density, and repeating a phrase unnaturally can count against you as keyword stuffing.
The useful question is simpler: does the page clearly use the words people search for, and does it read like a person wrote it? Seeing your most repeated phrases answers both.
Questions
What is a good keyword density?
There is no ideal number. Write naturally, use the main phrase in the title, the first paragraph and a heading, and use related words elsewhere.
Are common words like the and and counted?
They are left out by default so the useful words stand out. You can include them.
What is keyword stuffing?
Repeating words to manipulate rankings, so the text reads unnaturally. Google's spam policies name it explicitly.
Is my text stored?
No. Analysis runs in your browser.