Keyword density is a measure of how often a keyword or phrase appears in relation to the total number of words on a webpage. It is typically expressed as a percentage. The idea behind keyword density is that the more often a keyword appears on a webpage, the more likely it is that the webpage is relevant to that keyword.

It is important because in the past, some website owners would stuff their webpage with a keyword in an attempt to manipulate search engine rankings. As a result, search engines began to use keyword density as a factor in determining the relevance and quality of a webpage.

However, it's important to note that keyword density alone is not a definitive factor in determining a webpage's relevance or search engine ranking. Search engines use a variety of factors to determine relevance, including the overall content and structure of the webpage, the number and quality of external links pointing to the webpage, and the user engagement metrics.

Additionally, having a high keyword density can lead to poor user experience and can be considered as a spammy technique. It's important to aim for a natural balance and use keywords in a way that flows well with the rest of the content.

In conclusion, while keyword density is still used by some search engines as a factor in determining relevance, it is not the only factor and it is not as important as it used to be. Instead, it's more important to focus on creating high-quality content that is relevant to the topic and user intent, and that follows best practices for SEO.