From Gentoo Wiki
DRBD (or Distributed Redundant Block Device) is a network block device that provides reliability when storing data across multiple network nodes.
From the kernel documentation:
DRBD is a shared-nothing, synchronously replicated block device. It is designed to serve as a building block for high availability clusters and in this context, is a "drop-in" replacement for shared storage. Simplistically, you could see it as a network RAID 1.