It is hard to find a use for + operator in search engine. Here is an example that shows how exactly it works.
Search for a string
open apng file
You will notice that it does not pull relevent result. All it shows in PNG file result. Now change it to
open +apng file
Now you see fewer results but all has apng in it.
BTW APNG files are animated PNG files which ares not supported universally.
- Animate PNG files