What does Numbers 31:23 mean?

'Anything that can withstand fire must be put through the fire to be purified. But it must also be cleansed with the water used for purification. Anything that can't survive fire must only be washed with that water.' Numbers 31:23 - Modern Text Bible

(Items that can handle fire are to be purified by fire and water; others just by water. This is about making sure nothing unclean enters the camp.)