Welcome to Vodafone Community
There is some confusion with this, you have posted on the broadband board. As you would have received this by text I would be thinking along the lines of you being on a pay monthly or PAYG tariff. If you could confirm the price plan you are on this would get you the correct information.
edited. @SeanS To save the confusion I have moved the thread to the correct board. At a guess and not being PAYG myself, I would imagine you have exhausted you PAYG data allowance and Vodafone have offered an extra 500mb.
I would suggest you log onto your online account or My Vodafone application, this will confirm the remaining allowance. Alternatively, live chat or Customer Service can help but there will be a long queue. Follow this link to speak directly to the Social Media Team Contact the Social Media Team they will have the necessary account access and will be able to confirm the offer.