Yes, must be pretty hard to read for a starter, I think..
This expression essentially uses a set expression to create a kind of temporary selection in FIELD (only in the context of the sum() function), and using the sum function's result 0 or 1 as logic to filter the FIELD values in the application.
To understand set analysis / set expressions will probably take some time, but it will pay off. It allows quite easily to manipulate selections / sets and can be widely used in QV.
There are some advanced search terms that might be easier to understand, if you manage to read them correctly, like
='=wildmatch(Value,'&chr(39)&'1*'&chr(39)&','&chr(39)&'2*'&chr(39)&') and not Value=1'
should list all FIELD values that start with one or two except one itself. The chr(39) is needed to get the single quotes around the wildcard search terms '1*' and '2*' in the search string.