TOPIC: What is the travel sideways???

I can't completely understand the following sentence. Would you mind telling me what is "travel sideways" in the sentence below?
Lighting can also travel sideways for up to the same distance.


Thanks very much indeed.

About the meaning of " travel sideways" in the sentence below:
"Lighting can also travel sideways for up to the same distance,"
The opposite of " travel sideways"(horizontally) would be "travel up and down" (vertically).
I never thought about that! An interesting fact!!