The difficulty bomb is related to the switch to proof of stake. How does it work exactly and why is it going to help to switch to proof of stake?
The difficulty bomb is a part of the consensus algorithm; its goal is to make the difficulty of mining a block arbitrarily hard at some point in the future. When active, it will get increasingly hard for miners to create new blocks and therefore will get block rewards less frequently; which means less revenues. Exponentially. From the blog:
starting from block 200,000 (very roughly 17 days from now), the difficulty will undergo an exponential increase which will only become noticeable in about a year.
This is a very strong incentive to rapidly switch to the new network.