If you seperate words by " " as usual you get all results containing any of these words. You will get more results.
The search "offtopic fun" will be translated to "offtopic OR fun" an d requests all rooms containing offtopic or fun or both of them.
If you use "+" for separating words you only get results containing both keywords. Searching for "offtopic+fun" will be translatef to "offtopic AND fun". The number of results decreases.
You can mix these operators e.g. "offtopic+fun science" gets "offtopic AND fun OR science".
Now a result is in if it contains both of the words offtopic and fun. And it's even in if it only contains the word science.