The concept is closely related to keyword density, which is a measure of how often a keyword appears in a text.
However, there is no reason to become unduly worried about whether you are engaging in keyword stuffing. If your text is about a particular topic, you will of course mention it quite a bit.
When the use of the keyword becomes excessive, however, it can become harmful. It may, for example, mean that your page will not rank for the word in question, or that Google penalizes you in other ways. This is because keyword stuffing is often used in connection with black hat and grey hat SEO, which are more or less shady methods of ranking high on Google or other search engines. It is therefore a good idea to check whether your keyword appears too often. You can do this manually or by using an external tool.
Search engines have become so smart that they understand synonyms and prioritize texts that are written for humans.
First and foremost, it is important that you write texts that are intended for users rather than for search engines. You should, however, optimize the texts for search engines so that the search engines can understand their content and then rank them.
If you write texts about a specific topic, it can be tempting to mention the same word multiple times. In and of itself, that is fine, but you just need to be aware of keyword density, so the text remains easy to read while at the same time avoiding keyword stuffing. Synonyms and keywords related to the topic can therefore be an obvious shortcut to more readable texts that do not use the same keyword too often, and which are thereby more SEO-friendly.
