What is metadata API in Salesforce?

Hi,

Most applications manipulate data, which is often stored in a database. Typically this data is information such as Account details or Contact information. However, you can consider the source code for the application itself as data that provides the information on how your application looks, feels, and functions. You can refer to this information as metadata.

Use Metadata API to retrieve, deploy, create, update or delete customization information, such as custom object definitions and page layouts, for your organization. This API is intended for managing customizations and for building tools that can manage the metadata model, not the data itself. To create, retrieve, update or delete records, such as accounts or leads, use data SOAP API or REST API.

For more info you can refer to the Standard Salesforce Guide.