Core to the Salesforce infrastructure, Metadata is what makes the magic happen!
Metadata is a lightweight language based on XML. In Salesforce, metadata represents the structure of salesforce.com and Force.com development elements.
Basically, they are the core components or features that relate to the structure of objects and their fields, configurations, logic, code, and page layouts which build your look, feel, and the information architecture of your Salesforce Org. Instead of the traditional development approach, you can import metadata into your Salesforce Org and modify it in the product interface. This is super fast and immediately available through any web browser and there are no downsides to it