Organization-wide Defaults define the baseline level of access to data records for all users in an Organization.
Organization-wide Defaults are used to restrict access to data (Records).
Organization-wide Defaults (OWD) can be defined for Standard Objects and Custom Objects.
Organization-Wide Defaults, or OWDs, are the baseline security you for your Salesforce instance. Organizational Wide Defaults are used to restrict access. You grant access through other means we will talk about later (sharing rules, Role Hierarchy, Sales Teams and Account teams, manual sharing, etc).