# conspired against Nadab

"secretly plotted to kill King Nadab"

# Gibbethon

This is the name of a city. 

# Nadab and all Israel

"Nadab and the many soldiers of Israel" or "Nadab and the army of Israel" 

# were laying siege to Gibbethon

"were surrounding Gibbethon, so that the people of Gibbethon would surrender to them"