Kimmar's answer is correct but "just a drop in the ocean" not only means "a small part" but also "insignificant". I don't think it would be reasonable to describe Kimmar's "thoughts about language learning" as insignificant. The time to use "just a drop in the ocean (or bucket)" is when you want to disparage something as not making much of a difference. So... "even if you pick up all the garbage that you see on the beach, that's just a drop in the ocean considering how much trash is thrown on beaches everywhere". NOTE: Yes, I realize that "many drops make a river" and if everybody contributed just a little bit, the world would be a much better place. My example is simply to illustrate the use of the phrase "a drop in the ocean". The sentiment expressed in the example is not necessarily that of the author (me).

What does it mean? "Just a drop in the ocean" means just one tiny little bit of the whole thing. A single drop is compared to the whole ocean. I might say "My thoughts about language learning are just a drop in the ocean compared to my thoughts about life itself."

Another idiom with the same meaning is "a drop in the bucket".