Fiber optic cable is a glass type of cable that uses light to send data and other signals through it from one location to another location in a very fast way.
Just like when you typed this question and I type this answer the data you sent went through your internet connection and through the fiber optic cables along the way at lightning fast speeds.
The data went through lit fiber optic cable to reach the server that this website is hosted on.
Basically lit fiber optic cable is fiber optic cable that is active and has data being sent through the lit fiber all the time.
Dark fiber is just fiber optic cable that is laying in the ground that hasn't been activated with light signals to send data through it.
When fiber optic cable is laid down they lay down more fiber optic cable than they need because it costs less to go ahead and lay extra fiber optic cable down than it would be to lay more fiber optic cable later on when it's needed.
Thee are millions of miles of fiber optic cable laid down in the ground to transfer data through the internet and some of the fiber is lit up and active and some of it is just dark fiber and not lit up.
But someone can buy the dark fiber cheaply and light it up to send data from one data center to another.