Shard is a section of the cloud which is used for your account. Different accounts are on different shards. Which shard you get assigned to is a combination of the region you are in and when you created your Samsung account.
The universal sign on URL, https://account.smartthings.com , Will work for everyone because it looks up your account ID and then directs you to the proper shard. 
FAQ: How to find out what "shard" (cloud slice IDE URL) your Account / Location is on?