A destination ID is an identifier that represents the destination; i.e., the connected Google product, such as Google Ads or Google Analytics. The Google tag uses destination IDs to load destination-specific settings and to route events.
For Google Ads, the destination ID is the same as the conversion tracking ID, for example AW-98765.
For Google Analytics, the destination ID is the same as your web data stream's Measurement ID, for example i-gno/re-d12345.