Shields protect you from raids; stamina limits how often you can attack.
After your base is raided, a protective shield blocks further attacks for a window of time, letting you recover. The Shield Generator building strengthens your protection, and additional shields can be obtained from the in-game store. A shield cannot be activated while you are actively under attack, so plan your coverage ahead of your offline hours.
Each attack you launch costs 1 stamina. You hold up to 10 stamina, and it regenerates one point every 5 minutes (a full bar in about 50 minutes). If you run dry, you can wait for regen or buy an instant refill in the store for 150 SE. Stamina keeps raiding paced and meaningful — spend it on targets worth the energy.