Add-ons are customizations of a mod, sort of mods of a mod. While a mod modifies the main game, Minecraft, an add-on modifies the mod, i.e. Galacticraft.
Galacticraft has add-ons that add or modify the mod in many ways and degrees. Some may just add a new machine or modify a recipe, others add new rockets and entirely brand new galaxies.
You can check out the currently released add-ons in the Galacticraft Forum.