Know/learn/show (someone) the ropes
In all the variations of this expression, the ropes are the basic techniques that a sailor must learn about ropes and knots that were critical to seamanship in the days of sail. In a purely nautical context, these expressions must have been around for hundreds of years, but the current figurative meaning of understanding or getting familiar with the key aspects of an activity or business dates from the mid-19th century. Not to be confused with being on the ropes.