What is content:

• Content basically, all the plain text on your website is your content. Now search engines do look at your content especially the keywords you used in your content. It also looks at how many times you've used that keyword. 

• Example: If you're sites about "beauty-product" it'll look at how many times the keyword "beauty-product"  is used on your Web site. If you use the keyword only once in your content the search engine will think that the site is not related to "beauty-product".

• Another important point here is that the search engines do not only look for the keywords but also it's synonyms. This also helps your content look professional and doesn't make it feel like you're repeating one keyword over and over again.

keyword stuffing:

If you're thinking I'll just use my keyword in my article as much as I can and I'll rank better.

For example: purchase beauty products. Our products are best-selling beauty products.

Now this one line has more keywords than most of the articles do. But the people reading your article will hate it. Search engines will mark your website as spammy and unreliable.

So don't do this just let these keywords flow in naturally.