Shields

A shield can increase your character's defense beyond the protection their armor provides. Your character must be wielding a shield in one hand to make use of it, and it grants its bonus to AC only if they use an action to Raise a Shield. This action grants the shield's bonus to AC as a circumstance bonus until their next turn starts. A shield's Speed penalty applies whenever your character is holding the shield, whether they have raised it or not.
Raise a Shield is the action most commonly used with shields. Most shields must be held in one hand, so you can't hold anything with that hand and Raise a Shield, and you lose its benefits if that hand is no longer free. A compact shield and an irising shield don't take up your hand, so you can Raise a Shield with a compact shield or irising shield if the hand is free or if it's holding a light object that's not a weapon. You lose the benefits of Raise a Shield if that hand no longer meets these conditions. Irising shields and phase shields only count as a shield when deployed, as noted in their description. You can't Raise a Shield with an irising shield or phase shield unless it's deployed.
When you have a riot shield or mobile bulwark raised, you can use the Take Cover action to increase the circumstance bonus to AC to +4. This lasts until the shield is no longer raised or until any of the normal conditions that end Take Cover occur, whichever comes first. If you would provide lesser cover against an attack, having your riot shield or mobile bulwark raised provides standard cover against it (and other creatures can Take Cover as normal using the cover from your shield).
If you have access to the Shield Block reaction (from your class or from a feat), you can use it while Raising your Shield to reduce the damage you take by an amount equal to the shield's Hardness. Both you and the shield then take any remaining damage.