Bridge nodes push data from Ethereum into the Portal Network. They need a connection to an Ethereum full node.
The Portal Network is in development. One of the easiest ways to contribute is to help pump data from Ethereum into the Portal Network. The way you would do this is by running a bridge node. The more people that run bridge nodes, the faster Portal Network will be able to serve Ethereum's historical data and you can start using and building on top of super lightweight clients.