Addons are files that define one or more new soldiers, weapons, vehicles, structures or effects (or a combination of these) to OFP.
Addons are produced both by BIS themselves and by the OFP player community. BIS' addons are "official" addons, that is, they normally are or become an integral part of OFP's installation. Other people's addons are considered "unofficial".
To make use of a given addon, a reference to the addon must be defined in a mission, made by someone else or by yourself, should you ever become a mission maker. In other words, addons don't automatically become available for use in any mission you play. If you want to fly an F-18 Hornet in OFP, you have to have both the addon's file installed and a mission that was designed to incorporate the addon.
Addons normally come in PBO type files. They can be placed into one of several OFP AddOns subfolders. The best place to keep addon files is in mod folders.