In thematic editing, also referred to as montage editing, images are edited together based only on a central theme. In contrast to most types of editing, thematic editing is not designed to tell a story by developing an idea in a logical sequence.
In a more general sense, thematic editing refers to (as they say in the textbooks) a rapid, impressionistic sequence of disconnected scenes designed to communicate feelings or experiences.
This type of editing is often used in music videos, commercials, and film trailers (promotional clips).
The intent is not to trace a story line, but to simply communicate action, excitement, danger, or even the "good times" we often see depicted in commercials.