"To open the menu" means to access a list of options or choices that are available to you in a specific context, usually to make a selection or decision.
Sentences with «to open the menu»
Open the app, then slide open the menu on the left. (howtogeek.com)
To view all the screenshots you've taken, fire up the photos app, then slide open the menu on the left side of the screen. (howtogeek.com)
Fire that bad boy up, then open the menu by sliding in from the left side. (howtogeek.com)