I wanted to note that most American style guides use the em dash without a space on either side for interjected clauses and phrases—like this—while most British style guides suggest using spaced en dashes – like this – to do the same thing. Either way, they're different from a hyphen, which connects compounds, like "sixteen-year-old student".
Are you asking about the double dash "--" that appears after the word "painting"? If so, this is called an "em dash" and it is used to set off a clause. Many times it appears as a single, long dash, however, if the font cannot accommodate a long dash, it may appear as two shorter ones.
Typically, an em-dash sets off a clause that could be left out of the sentence but is closely related and gives extra information, like this: The traveler studied the signpost -- dirty and hard to read -- then decided to take the path to the right.