Fittings are items that you can install to get a bonus of any kind. Each ship has six (6) permanent fittings, two (2) for either sails, guns and hull. Permanent fittings are lost upon sinking (and provide insurance if you bought it) and are also lost if you throw them away.
Besides permanent fittings, each ship also has three (3) general fittings. Unlike permanent fittings, these are NOT lost upon sinking, however still lost if you throw them away.
Try to avoid Stacking Penalties by not installing more than two (2) items that provide the same stat (bonus). For example, it's generally better to install one (1) Double-Planked Armor and one (1) Heavy Planked Armor, instead of two (2) Double Planked, unless you prefer a short-range fight. The same applies to other stats (bonusses).
This information is provided by the update that PotBS Wikia is undergoing, by Captain Vuur.