# Your gold and your silver have become tarnished

"Your gold and silver will become tarnished" 

# have become tarnished ... their rust

"are ruined ... their ruined condition" 

# their rust will be a witness against you. It

"and when God judges you, your ruined treasures will be like someone who accuses you in court. Their corrosion"

# will consume your flesh like fire

Here "flesh" stands for the physical body and corrosion is spoken of as if it were a fire that will burn up their owners. The idea of fire is meant to lead people to remember that fire often stands for God's punishment that will come on all the wicked. 

# for the last days

"for when God is about to judge you"