It is the percentage of no. of times your keyword appears within a post. Here is the formula:

(No. of time Keyword Apearing in Content / Total Word Count) X 100

An ideal keyword density should be 2-3%

use the target keyword for 12 to 14 times in your content. It is best to reach the decent keyword density. best use the combination of the target keywords to avoid the repetition.

The keyword density can be between 2-3%. Having a page with keywords more than this percentage can lead to penalty due to keyword stuffing.