If you notice that your money is worth more tomorrow, you are going to save your money and spend it tomorrow. This is true in situations of "deflation".
This sounds good to you, because all you have to do is wait and you'll be better off tomorrow. You don't even have to work to get more stuff.
However, you aren't the only person with this idea. Everyone saves their money in the bank. The people who sell goods and services, sell fewer goods, and maybe go out of business.
The economy slows down.
On the other side, if you design an economy so that inflation exists. People want to spend because in the future their money will be worth (a little bit) less.
Money flows around a lot. People who sells things make money and stay in business. People have jobs and the money is still worth something.
Remember money isn't worth something, trading good fluidly and efficiently is worth something. Money lets us do that. So a currency that lets us do that is good.
If anyone knows more about the situation, please add it. I don't know any economists so this story is probably pretty incomplete.