What is the keyword density in seo?

I am going to do onpage optimization for my new project.i am new to SEO field.please anyone tell me what is keyword density? keyword density really important for onpage optimizations?

We already have a number of threads about keyword density and related topics, so if you do a search of the forums, you’ll find plenty of information. A good place to start might be here:

After that, if you have a more specific question which has not been answered, please feel free to post again. Meanwhile, thread closed.