Good question!
A pre-sale allows potential investors to get in early enough in the sale at a discount price.
Usually 10-25% off the original price of the tokens when the pre-sale ends of the ICO and the regular sale begins.
They set aside enough for the pre-sale so this the reason why they offer this bonus and usually with some perks.
You can see those for the one in my singnature if you want an example of one currently running it's token sale in it's second phase of the pre sale along with bonus discount for purchasing them early before the sale of the tokens end.
Thank you
dashingriddler
Ambassador for the Centra product line.