What does that sentence mean? Obviously, most people who know English would also understand each word in that phrase. The words are not difficult, and the structure of the sentence is not difficult.
But, what does it mean?
We cannot decide what the sentence means until we know more about the context. Is the person trying to do something in a dark corner? Is she holding a cigarette? Did he just walk into a bar?
Of course, in this simple example, the ambiguity is caused by the range of meanings (and, in fact, different meanings) of the English word “light.” When you add even more words with even more possible meanings and even different possible grammatical and syntactical connections, ambiguity is increased when trying to determine what a particular sentence means.
So, how do we know what sentences and words mean? We can only determine the meaning of sentences and words by examining them in context.