Some say the world will end in fire,
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
In the previous line, Frost claims that he favors those who favor fire. But if he had another choice, his second option would be ice. 
 
 Frost continues with the theme of reconciliation of opposites. Opposites in this situation being fire vs. ice.
Fire and Ice