Salesforce data specifically refers to all your business data residing inside of Salesforce. This includes Accounts, Contacts, Leads, Opportunities, Cases, Contracts, custom object records, files, attachments and much more.
Salesforce metadata refers to all your configuration settings, such as custom fields, reports, dashboards, page layouts and custom code like Apex and Visualforce. It also includes the interdependencies and hierarchy of Salesforce data. For example, an Account has certain Contacts, Opportunities and Contracts connected to it.
Think of Salesforce metadata as the glue that connects everything together. When something is deleted, lost or corrupt in Salesforce, so is the metadata. So, when restoring Salesforce data, it is critically important that the metadata is also restored and reconnected. For a salesforce backup solution to be effective, it must also be a metadata backup solution.