Ha ha, I love this.
As others already mentioned that is the brake caliper bleeder screw which is used to drain fluid and air out of the brake lines; all hydraulic braking systems have these.
Hopefully you just rotated the rubber cap, which likely did nothing.
Curiosity and fiddling is how I've learned pretty much everything in my life... However always do lots of research when fiddling with your motorcycle because there is not much room for error.
2008 Versys 650
Been riding 8 or 9 years, and have owned 8 or 9 bikes; its an addiction I can't quit...