Clicking the checkbox provides a truncated search for the inserted characters.
The default setting is an empty checkbox meaning the search is "only" for the entered term.
With the checkbox checked a truncated search is sent to the server additionally searching for none, one or more characters before and after the term.
Search for "fluor" will find "Aluminiumfluorid, Bariumhexafluorosilikat and more.
Also finds Fluoranthen, Fluorethan and more..
Notice: A truncated search can generate many more hits than a standard search. It might take a slight moment for the results page to display.