The books say your espresso machine is worth $30,000 (what is left of its cost after depreciation). One day it half-breaks, or a new model makes yours obsolete, and realistically it is worth $10,000. Accounting requires you to admit that: you cut its book value by $20,000, and that $20,000 hits the income statement as an expense. That is a write-down, officially admitting something you own is worth less than the books claim.
The timing game: when to admit it is partly a choice. A manager having a terrible year anyway might dump every admission into it, the broken machine, the stale inventory, all at once, making one year look catastrophic so the following years look like a heroic recovery. This even has a nickname, "big bath" accounting: take all the pain in one bath.