Dark fiber is bare optical fiber, leased “unlit” (with no active equipment on it): you light it with your own transmission gear (DWDM, Ethernet). You then fully control the link's bandwidth, latency and security.
In practice
It's used to link two data centers, a site to a POP, or to build your own network. It contrasts with a “managed” link (turnkey guaranteed bandwidth). Dark fiber offers near-unlimited capacity but requires managing the optics.