Understand how to do something; have experience of the appropriate procedures
It seems obvious that this expression derives from the need to sailors to know how to tie knots to secure rigging on sailing boats. It may well have a nautical origin - sailors certainly did have to learn which rope raised which sail and also had to learn a myriad of knots. However, there is also a suggestion that it comes from the world of the theatre, where ropes are also used, to raise scenery etc. (https://www.phrases.org.uk/meanings/know-the-ropes.html)