system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does the US Federal Reserve consider a 2% rate of inflation to be the sign of a healthy economy?

NPR today was discussing the US Federal Reserve and the fact that they were unhappy that the rate of inflation was below 2%. Why?
gpt
Negative inflation is a disaster for an economy. 

Let's say you have a dollar: in a low-inflation scenario (ie, the economy we have, around 2%) you can invest it and risk losing it but it might gain value, you can put it in a bank and get a little interest to balance out inflation but you probably won't lose it, or you can put it under your mattress and it will gradually lose more of its buying power. This encourages you to save in banks and investments where your money will go into the wider economy and circulate. Banks make safe loans hedged by spreading them out between a lot of people (ideally, anyways - of course the financial crash showed that wasn't always true) and investments put money into businesses at the risk of the business failing, but either way the money keeps circulating around. 

Now, consider a scenario with deflation. If you stick it under your mattress, it will gain value with no risk of losing it; if you give it to a bank, they might have to charge you to hold onto it, because nobody would borrow money from a bank then unless the interest rates were nearly zero or negative, and investing is pointlessly risky since you risk losing all your money. That means everyone else will probably stick their money under their mattresses too, which means there isn't any money available to spend anywhere in the economy... which further increases the value of money relative to goods, and encourages more people to take their money out of the economy. That continues to get worse until either the government prints more money, or the system collapses. 

High inflation is bad too, for slightly more obvious reasons - if money loses value fast, the cost of borrowing gets higher, prices on everything go up, take-home pay becomes worth less, etc... but it tends not to turn into a self-reinforcing spiral. So, ideally we want inflation to stay at low, safe, predictable levels.