Wargear in Dawn of War 2 is any item or weapon that can be equipped to change, increase, or otherwise modify a unit or commander's combat effectiveness. Wargear can be changed (usually at a cost of Requisition and/or Power) in the small pane to the left of the unit's portrait in the bottom right of the screen.
Commander wargear is separated into 3 categories, Weapon, Armor, and Accessory, respectively from left to right, sorted by columns in the commander wargear pane. Only one of each type of wargear can be equipped on a commander at one time. If at any point one chooses to switch to a different wargear in the same category, the old wargear can be re-equipped at a later point in time at no cost. Previously equipped wargear is denoted by a small green check mark on the icon. While there will be no Requisition/Power cost, it will still take the same amount of time to re-equip.
Regular unit wargear is permanent and usually exclusive. For example, the Tyranid Warrior Brood has three different types of wargear available to it, each with very different utilities. Each is permanent, requiring a new Warrior Brood to be created in order to choose a new set of wargear. An example of wargear choices that are not exclusive is the Nob Squad. The Nobs have 3 total upgrades available, each of which stack and grant different effects.