The underlying of the hashrate option is the block difficulty. Each block in Ethereum has a difficulty field and for any chain we can calculate the total difficulty of the chain by adding difficulties of all the blocks from genesis to the head block.
Block difficulty fluctuations are caused by a few main factors:
technological progress (hashrate growth resulting from faster GPUs and ASICs)
miners joining or leaving the Ethereum mining market
changes to the calculation methods of difficulty
When buying or selling a difficulty option you are taking a view on one or all of the above - you may expect some sudden difficulty change due to a new ASIC introduction (technological progress) or some network upgrade to happen, or miners exodus or influx cause by the economic of mining on Ethereum vs other protocols.
Historical block difficulty chart (see etherscan.io for the current values):
Block Difficulty changes since the Ethereum genesis block
Note that hashrate can be mostly expressed in difficulty but it is an approximation (we need to exclude changes to the calculation methods).
Historical hashrate chart (see etherscan.io for the current values):