The number of block solutions computed per second by all miners on the network
Because the Hash Rate requires real world computing power and resource investment that cannot be faked, this chart also represents technical and monetary investment in the infrastructure of Bitcoin.
A cryptographic hash function takes digital data of any size as input and produces a random (but fixed-size) string of digital data as output. Files, poems, pictures, the entire written works of Tolstoy, or any other digital data could each be hashed with SHA-256 to produce a unique 256-bit output called a "digest."
Hashes are a type of one-way function because it's not possible to re-create the original data from the output digest. However, it's easy to prove a given input data results in a specific output digest by performing the hashing calculation on the given data and confirming a match to the output digest.
In Bitcoin proof of work, miners use the transactions of a block and other special identifying data as input to the SHA-256 hash function. To "mine" a block, miners must discover a block that hashes to a digest with a certain number of leading zeros. The output is random and unpredictable, so the hash calculation must be performed several times while slightly changing the input data until a valid hash is calculated.
The Bitcoin network is designed to always take about 10 minutes to discover a valid hash no matter how much hashing power is added to the network. As more computing power is added to the network, valid blocks require hash digests with more leading zeros (the difficulty).
Today, Bitcoin's Hash Rate has grown to Exahashes per second, or 1,000,000,000,000,000,000 hashes per second.