Raisin's are a dried fruit made from black grapes.
They contain about 70% sugar so contribute towards the beer's gravity.
When using raisins in beer it's best to add them after primary fermentation, when the wort's better equipped to deal with any risk of infection.
To prepare for use, boil water and pour over the fruit before leaving for around twenty minutes. Not only does this pasteurise/sanitise them, it also softens the sugars in preparation for the fermenter.
If you want to extract even more flavour, I've read that making a purée from this softened fruit can help.
However, in my experience the effect of the raisins is far from subtle. The flavours and smells are strong even when added whole.