A RouteTable defines a set of rules which determine where traffic is sent. They can be defined at the level of:
By default all Vpc start with a single table the main route table, this is used by all subnets not associated with a custom route table. The main route table allows traffic to flow between all subnets within a VPC.
Custom routes can be setup to send traffic to other locations, e.g. a Virtual Private Gateway or Internet Gateway, by creating custom route tables and associating them with a subnet instead of the main route table.