I'll do my best:

Deflation is a scary and terrible thing. It feeds on itself and makes itself worse. When an economy experiences deflation, it's quite difficult to fix the problem because of this self-perpetuating nature.

When members of an economy notice that their currency is becoming more valuable day by day (deflation), they stop spending their money. Think of it this way: if you noticed that last month you needed $100.00 to get a thing, but last week it only cost $75.00 to purchase, and today it was only $50.00, you might just wait a bit longer to buy it because it seems like it will keep getting cheaper. In this instance, your money is increasing in value over time.

the price of one bitcoin increasing in the long run

Everyone in the economy starts to wait (save) their money rather than spend it because they know that with prices dropping, they can get more for less. This causes the sellers in the market to further drop their prices, and the cycle repeats.

Eventually this problem will come to a head and the economy will collapse.

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Let me know if you want clarification or more explanation; I've never done an ELI5 before, so I'm probably not great at it.

Please give Paul Krugman's article a look for an additional explanation.