What is Custom metadata in salesforce ?

Custom Metadata Types are similar to Custom Settings in Salesforce, except that they are deployable and packagable.
To create a Custom Metadata Type, navigate through Setup and click New. Similar to the creation of a custom object, you are prompted to add a name and plural name, API name, description and visibility. From there, you can create the custom fields required to meet your use case.
There are several ways to use the data in a Metadata record. For developers, Apex can access the data as long as the metadata records are visible in the code’s namespace. The syntax looks like this:
MyCustomMetadataType__mdt custMeta;