The percentage of keywords in your content. The percentage of repeating keyword in a 250 words article is 3-4% and if you exceed the limit then it would be counted as keyword stuffing.
In simple Keyword Density can be defined as the "no. of keywords/total no. of words on page*100". Hope you understand it easily. But you keep watching that keyword density would not increase by 4% - 5% on your web page. Otherwise it will produce a negative effect for your website.
keywords/total words*100 = keyword density You can use 4 or 5 keywords in 100 words of content but exceeding limit can effect negatively.