Hash rate measures how many attempts per second the whole network makes at finding a valid block. It is quoted in terahashes (TH/s), petahashes, or exahashes (EH/s, a quintillion per second).
Higher hash rate means a more expensive chain to attack, because rewriting history requires out-computing everyone else. Falling hash rate can mean miners are switching off unprofitable machines, which traders watch through hash-ribbons and miner-capitulation.
Example: at 600 EH/s, a 51-percent-attack requires marshalling over 600 EH/s of your own hardware. At roughly 20 joules per terahash, the honest network alone burns about 12 gigawatts, which is the scale an attacker would have to match.
Related: proof-of-work, mining-difficulty