normal wipe (format) just deletes the data.
secure wipe deletes the data and replaces it with 0's . Usually 7 cycles of writing over the data is the best option, but you can do less. It's done because generally speaking you can recover the data if its just wiped. So by wiping it, then replacing the data with 0's and wiping it and repeating the process a few times eliminates the chance someone could recover the original data you deleted.