Mikveh Dishes

We received gifts recently of a glass Shabbat candle holder as well as a cake dish also made of glass. My neighbor told me that I need to take them to the mikvah for dishes. I didn’t know there was such a thing nor why I need to take them for a special dunking in the water. If it’s for dishes, then why take the candle holder – it won’t be used for food or drinks? I don’t get it.