There are lots of ways you can contribute to WordPress. Many people that use it contribute in some way, even if it’s simply reporting a bug. I’ve contributed by reporting bugs, editing and improving documentation on the Codex, helping out in the support forums, fighting for what I think is right on Trac, giving away free themes and plugins, and probably other things I’m forgetting right now.
Now I’m a “core” contributor.
This means that code I wrote was accepted into WordPress itself and is now released to the public. That code is now running on over 1.6 million sites at this time of this post. WordPress 4.4 was released yesterday.
That number will continue to increase for years. Pretty cool.