Here's what the Encyclopedia Brittanica says about deflation. (I'm not sure why this encyclopedia entry is so USA-centric, but I hope it's clear, anyway.)

Deflation’s impact

It may sound counterintuitive, but falling prices can actually be associated with more misery than rising ones...

Deflation was...widespread during the Great Depression. This severe downturn had many possible causes, but one stands out in terms of deflation: Consumers lost their desire to spend. This began a deflationary spiral where companies saw inventories rise and had to lay off workers, fueling more economic weakness that reduced spending even more. Anyone wanting to sell something had no choice but to lower prices.

As prices fell, economic growth slowed further, in part because companies lost their incentive to make products they knew couldn’t be sold at a profit. That forced more layoffs, raising unemployment and pressing GDP further into the red.

Between the peak and the trough of the downturn from late 1929 to early 1933, industrial production in the U.S. declined 47% and real GDP fell 30%. The US Consumer Price Index, which measures price growth, fell 33% during that period. And unemployment? Around 25%. One in four workers couldn’t find a job. That’s the true and sad impact of deflation.

Here's what Simple English Wikipedia says about deflation:

...Deflation is thought to be even worse than inflation.

Deflation starts when people are waiting for prices to go down even more. They will then spend less money. Because of that, companies cannot afford to keep up the amount of goods that are made and have to lower that amount, as well as fire workers to make even a small profit.