The external sharing features of Microsoft SharePoint let users in your organization share content with people outside the organization (such as partners, vendors, clients, or customers). You can also use external sharing to share between licensed users on multiple Microsoft 365 subscriptions if your organization has more than one subscription. External sharing in SharePoint is part of secure collaboration with Microsoft 365.
See this link for more information (including directions on how to enable per site collection instead of tenant-wide).
If you want to get straight to setting up sharing, choose the scenario you want to enable: