I usually ride as far to the right as is reasonably possible, anyway, so if I hear it, to me it means "I'm coming up on your left, now is not a time to veer over or decide to make a sudden left turn". If I were riding two abreast, I would switch to single file, or closer together, whatever I thought made the most sense in the situation.

The fact that that person would speed up is petty and rude. The fact that he would actually move over and actively block you is even worse. I'm appalled that someone would act like this.