I would assume you're talking about the " air box (oil) drain".

Forgive me my lack of technical detail but afaik it works a bit like this: When your engine cylinder is pushed downwards, it pushes away all the air ( and oil) underneath it. This pressure build up has to go somewhere so this air/oil mixture goes to your "breather". A little black box that seperates this mixture again and allow the oil to flow back in to the engine compartment. However, when revs remain high for a while ( long motorway drives?), this breather can't hold it all and it "overflows" into your "air box" where your air filter is. If you open up the air filter box, you could be able to see the entrance of this tube. Normally the air/oil would be sucked again in the engine and burned. However a little bit can stay behind and drip down. Hence, a little drain tube...

Should you be worried? No ...

Should you look at it from time to time? Yes of course.

for example:After a 2000km holiday ( in 6 days) with lot of constant high revs, I lost about 5 centilitres.. With normal rides, it's hardly worth mentionning