To produce CO2, the bubbles we enjoy when we drink our beer, we need to add a small amount of sugar. The yeast eat this, and produce carbon dioxide.
The source of the sugar can vary. Possibilities include dextrose (corn sugar), sucrose (table sugar), corn syrup, malt extract, honey. Opinions vary on which is best, and, um, I don't have one. Use what you feel most comfortable with.
There is also a difference of opinion on when you add the sugar. Either add it all at once in a separate bucket, then fill the bottles; or, fill all the bottles, then add a small amount to each bottle. I've done both, and I can see advantages to each.