Living in Leiden, you might as well want to join the amsterdam meetups directly, as it is closer from you.
The idea is quite simple : meet awesome people, discuss with them, share experience and be amazed by all the good energy that can come from all those awesome people.
I joined some meetups already in Utrecht (Ruby meetup, Startup Weekend) and always get out of it feeling like working all night. . . just to be a bit as amazing as the people I can meet.
I have a lot of side-projects/ideas but always struggle to find "real" people to share them with. Meetups are a really good way to find people that have the same kind of interests as you :).
And it is totally awesome to meet strangers (even though a bit stressful). Always good to get out of your comfort zone from time to time :).