What does this diagram mean, in English?
There are two separate text labels there, which apply to 2 separate wires.   

One of the labels is "switch/black" which seems wrong. It seems like it ought to be describing the switched-hot wire going up to the fan, and black is at best a guess which color it might be; red is likely.   The other label is "switch/red" which seems right. 

The symbol is not part of the text labels, it is part of the diagram.  It  is meant to be a wire nut.  

The labeling and instructions are part of the UL approval which results in a UL listing, and it's a surprise to see such an amateurish mistake.  Are you sure this thing is actually UL-listed (or CSA, ETL or other comparable NRTL; note that CE is not one). There is a lot of cheap Cheese junk being offered on Amazon Marketplace, eBay, banggood and other purveyors of the junk from Alibaba; most of this has the ill-reputed CE mark if it has any markings at all.  These cannot be attached to mains wires in the US or Canada, NEC 110.2 "Equipment must be approved" (by UL or other NRTL).