Quite often you can figure out the essence of the sentences even if you don't know all the words. Of course, you do need to have a basic vocabulary. While speaking/writing it is possible for you to convey what you want to without using complicated words. If you can simplify your thought and strip out all that's non-essential, you will be able to convey the essence of your thought.

However, if you don't have a grasp of the basic grammar you're prone to confuse the person you're talking to. If you're reading, then you lose the context.

Exceptions abound, but I have found this to be true quite often while studying French.