This is a very difficult question. The easy answer is they are equally important. If I had to choose between the two, I would say words are more important than grammar, but you must have some knowledge of grammar to make yourself understood. Quite often, a native speaker of a language can understand what you are trying to say if there are many grammar mistakes but the words are correct. If the grammar is perfect but the words are all wrong, there is no way to understand. TRY YOUR BEST!

I think having a basic understanding of grammar is good but, for me, having a strong vocabulary is critical. Great grammar with a small vocabulary is like knowing how to build a house but not having any tools or building materials. The knowledge then becomes useless because it can't be applied practically.