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My Domain is a Salesforce feature that lets you highlight your brand, makes your org more secure, and allows you to take advantage of some new features of Salesforce like Lightning Pages, the Lightning App builder and, what we are interested in most, Lightning Component tabs.
The easiest way to see if you have My Domains enabled is to log into Salesforce and look at the address bar. If you see something like https://yourcompany.my.salesforce.com, then you are all set and can skip to the next step. However, if you see something like https://na31.salesforce.com, you do not currently have My Domain enabled and need to take a few extra steps first.