# 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.

# tar

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

# the kings of Sodom and Gomorrah

the kings of Sodom and Gomorrah and their armies" 

# they fell in there

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 were left

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