What does (+) mean in SQLPlus?

I just stumbled across a SQL+-Query which uses (+). I've never worked with SQL+ before and I've never seen something like this. I tried to ask Google about it, but I couldn't find anything useful since Google obviously filters the "(+)" and just ignores it...

Example:

[...]
where [...] 
    AND 16791688 = T7mm.child_fielddef_id  (+)  
    AND T7mm.parent_dbid = T7.dbid  (+) 
    AND T1.dbid <> 0 [...]