The Summary tab provides a high-level overview of data surrounding the route table, detailing which VPC it resides in, the account owner, the route table ID, and any explicit associations, as can be seen here:
These explicit associations relate to any subnets that have been configured to use this route table using the Route Table tab within the subnet configuration. Whenever you create a new subnet, it will automatically, by default, use the main route table created by your VPC. However, as I mentioned earlier, you can create different route tables. If you then configured your VPC to use one of these new route tables, then it would be implicitly associated.
