Deflation will be bad because nobody will by something today when they think it will be cheaper tomorrow.
Deflation is the opposite of inflation. One is an increasing value of currency, and the other is a decreasing value of currency.
People deal with deflation every day - both in their own national economies and internationally - when your currency is rising (gaining value) why would you purchase something today when it is going to be worth more tomorrow? The answer is because the value of having the thing today is worth more than having the money tomorrow - which is how we base all our purchasing decisions.
It's true that deflation will have effects that are not good - but mild deflation is no horrific thing. Most people have been told deflation is a scary monster in the same way that they have been told marijuana is a scary drug. There are circumstances and reasons when that is true, but it's not generally true and generally overblown. Both high deflation and high inflation are runinous to an economy, but mild deflation and mild inflation just produce different classes of winners - savers vs creditors.
When an investor chooses whether to invest in something, the question they ask themselves is "will this make more money than the best alternative investment on offer to me?"
When the value of currency is falling, it becomes relatively more appealing to invest in things that are not currency. For example, new machines for the local factory - those machines make widgets, and with 2% inflation those widgets are worth 2% more every year.
When the value of currency is rising, it becomes relatively more appealing to invest in things that are currency, like keeping my money in the bank. Glad I'm not one of those suckers who invested in the widget factory, because of the 2% deflation those widgets are worth 2% less every year.
Me, I think our economy would be better off with more invested in manufacturing and less in financial services.
Also, the value of your loan increases in real terms, as does the labour-share of input costs (because the workers aren't going to give back wage gains in times of deflation). Your return is hampered.
These are some of the problems with deflation.
Savers and creditors are the same people.