What is an RPC endpoint?
To connect to and retrieve data from a blockchain, you'll need to connect to a node on the blockchain network. A node is a participant in the network that stores and serves the latest blockchain state. Users who wish to download blockchain data can either run a node themselves, or connect to a publicly-provided node via the node's RPC endpoint. An RPC (remote procedure call) endpoint is like a node's address: it's a URL which requests for blockchain data can be sent to.
By default, the Dark Forest webclient will connect to https://rpc-df.xdaichain.com/, a public endpoint provided by the xDAI team. xDAI maintains a fleet of nodes which respond to requests made to this endpoint. There are other public endpoints you can connect to as well; if you'd like a dedicated node to serve your requests, you can also run your own xDAI node and connect to the endpoint it exposes.