Moneropedia
Bootstrap-node
The Basics
The daemon running on a local node has to sync with other remote nodes. While it is not fully synced, wallet may still be connected to the local node. Therefore, the wallet cannot access the blocks that are not yet synced on the local node.
To allow the wallet to be immediately usable, the daemon on the local node uses a bootstrap node to which the RPC request are proxying to, giving access to the missing blocks.
Note: the replies from the bootstrap node may be untrustworthy.