Translation:I am watching the boys playing tennis.
In Russian, you typically need a word connecting a subordinate clause (мальчики играют в теннис) to the main clause (Я смотрю). There are a few cases where you can get away without such a connecting word, but you are still better off using one (at least as a beginner), and this is not one of those cases anyway. So "как" is just such a connecting word here, it does not carry the actual meaning of "how" - although it certainly can. Whether or not "как" is 'loaded' with this meaning would generally depend on the context. This sentence is neutral enough, so I would not generally expect such a meaning here, and if I wanted to convey it, I would actually modify the sentence as I described earlier.