Keyword nets are best described visually.

Given the keywords, "Personalized", "Gift", the strongest match for relevancy is going to be one or both of those keywords ONLY. We call the strongest match the "Tight Keyword Net".

The Wide Keyword net catches more keywords, just not as strong of a relevancy match. Given the keywords, "Personalized", "Gift", you can see that "Birthday Gift" includes one of them but also a non-targeted keyword so it's a weaker relevancy match.