Help with .open (css)

So i´m making my portfolio and i want to make my own responsive nav bar without any framework i searched the web for some basic tutorials and i found an easy one that i understood everything except:

    #nav-trigger {
      border-left: solid 30px transparent;
      border-top: 0;
      border-bottom: solid 30px #000;
      border-right: solid 30px transparent; }

.open is not a class in the html doc so what is it? how can i search more on the web about it? its a css behavior? that’s the only thing i didn’t understood

i´m sorry if my question is dumb i´m just getting started (superNoob)
thanks for your time.

for more context this is the tutorial i used

It is a class added/removed by jQuery (see “Some Notes About The Javascript/jQuery”).

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Thanks for your time i see it now