What is keyword stuffing?
Keyword stuffing is the practice of overusing a keyword to try to manipulate search rankings. Google's algorithm detects and penalises this. If your focus keyword density exceeds 3–4%, you should rewrite some sections to use natural synonyms instead.
What are 2-word and 3-word keywords?
These are bigrams and trigrams — phrases of 2 or 3 consecutive words. "Digital marketing", "content management system", and "search engine optimisation" are examples. Tracking these is important because most modern searches are multi-word phrases, not single keywords.