Weapon slots are specified in a hull's .ship file and determine where weapons can be mounted.
A unique identifier for the weapon slot. Used in the .variant file to identify which slot each weapon should go in.
X/Y coordinates of the mount.
Default heading of the weapon.
The angle in which the weapon can pivot from left to right (so 120° arc means the weapon can turn up to 60° in either direction).
Mount type: turret / hardpoint / hidden.
Turrets can rotate around large arcs. Hardpoints are generally fixed to 5 or 10 degrees and rotate more slowly than turrets. In vanilla, the same weapon will have a different sprite when placed in a turret vs. a hardpoint. Weapons in hidden mounts are not rendered (except for visibile missiles) and cannot be disabled.
Small, medium or large.
Ballistic / Energy / Missile / Hybrid / Composite / Synergy / Universal / Launch Bay / System / Decorative / Built_In / Station_Module. Pretty self explanatory, determines what type of weapons can be slotted in.
Launch Bays are where fighters are launched and refit from. System mounts are from where certain ship systems like EMP Emitter and Flares are fired. Built_In weapons are specified in the .ship file and cannot be replaced or removed. Station_Module slots are where modules are attached to stations or ships.
See Refit Screen for information on weapon types. Note that while a weapon can technically be assigned the Hybrid, Composite, Synergy or Universal weapon types, such weapons can only go into Universal Mounts (so e.g. a Hybrid weapon cannot fit into a Ballistic, Energy or Hybrid mount).