Now

This word introduces background information about the valley of Siddim.

was full of tar pits

"had many tar pits." These were holes in the ground that had tar in them.

a thick, sticky, black liquid that comes up from the ground.

the kings of Sodom and Gomorrah and their armies"

Possible meanings are 1) some of their soldiers fell in the tar pits or 2) the kings themselves fell in the tar pits.

"Those who did not die in battle and did not fall in the pits"