A destination ID is an identifier that represents the destination; i.e., the connected Google product, such as Google Ads or Google Analytics. These IDs are primarily used by the tag to load destination-specific settings and to route events.
For Google Ads, the destination ID is the same as the conversion tracking ID. For Google Analytics, the destination ID is the same as your web data stream's Measurement ID.
Examples: G-12345, AW-98765