Standard objects are objects that are included with Salesforce. Common business objects like Account, Contact, Lead, and Opportunity are all standard objects.

Trailhead – https://trailhead.salesforce.com/en/content/learn/modules/data_modeling/objects_intro

Reference Link – https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_list.htm