Hash Rate is the number of hashes that are being calculated every second while mining bitcoin. the problem is that miners don't report how many hashes they calculate, we don't know how many miners there are, blocks don't store "h/s" anywhere, and we have no way of actually measuring it.
what we can do is to come up with an estimation of the rate based on difficulty and the time it took to find certain number of blocks [1]. it is an "estimate" because the time between blocks is not fixed, it is random due to the chance-based nature of finding a hash that satisfies Proof-of-Work condition. and also difficulty is fixed until 2016 blocks are found. so when you do the calculation in a shorter period of time you sometimes end up with big ups and downs.
another interesting thing is that if you do the calculation on your own and choose 2 random times as the day's start and end (like from 2:35 PM yesterday to 2:35 PM today) you end up with different values.
in other words this chart:
https://www.blockchain.com/charts/hash-rate?timespan=60daysthat is being advertised today as the cause of the "drop" is nonsense. if you want to look at a better hashrate estimation closer to reality look at this chart:
https://www.blockchain.com/charts/hash-rate?timespan=60days&daysAverageString=14
it is the average in 14 days which is teh difficulty adjustment period and since the time frame is bigger
the error in the final calculated value is smaller hence the smoother chart instead of zig-zag-y one.
of course there is always hashrate fluctuations as miners come and go and it is visible on this chart too but nothing drastic as they are spreading all over the internet today!
[1]
https://en.bitcoin.it/wiki/Difficulty#What_network_hash_rate_results_in_a_given_difficulty.3F
