Raid Shields allow you to protect your trophies and resources for a brief period of time. 
You can purchase Raid Shields though the in-game Shop, under the Resources tab. 

Your Raid Shield is active for the duration of the purchased amount. However, if you initiate a Raid, the Shield will be broken.  

After your Raid Shield has expired, a cooldown with the same duration of the purchased Shield will take place.

If you prematurely break the shield by initiating a raid, there will be a cooldown for as long as the shield was active. You can then buy a new Shield.