For example: a website contains the keywords that make up the search term “dentist Boston implant” in the heading “Your professional dentist in Boston; dental practice for minimally invasive implants”.
The search term proximity between “dentist” and “Boston” is one word, between “Boston” and “implant” it is five words (Boston; dental practice for minimally invasive implants).
The smaller the distance between a search term’s individual keywords, the more relevant it will be from a search engine’s point of view.