Keyword density refers to the percentage of times a keyword appears within the content compared to the total word count. However, there is no fixed ideal keyword density for on-page SEO. Instead of focusing on a specific number, it is recommended to prioritize natural and organic keyword usage. Keyword stuffing, where keywords are unnaturally repeated to manipulate search rankings, should be avoided as it can have a negative impact. The emphasis should be on creating high-quality, informative content that naturally incorporates relevant keywords to provide value to the readers and search engines.