WHAT IS IT

It's a digital currency, and a store of value, that exists on the Bitcoin blockchain (as opposed to say, an ethereum blockchain). It is not owned by anyone or controlled by a central bank. All you need is a bitcoin wallet address and you can send and receive bitcoins to anyone around the world in minutes at near 0 cost.

THERE IS A FINITE SUPPLY, ONLY 21 MILLION BITCOINS

This is awesome for 2 reasons. In our currency money, central banks print money. When they do this a lot, it reduces the value of the currency - this is called "debasement". Ie where it used to cost $2 for a bar of chocolate, and the government prints a whole lot of cash, it then might cost $3. One of the (many) broader implications of this is reducing the value of people's savings accounts. 

Extreme examples of printing money causes "hyperinflation", we see this in countries such as Venezuala where it is cheaper to burn paper money than it is to buy firewood.

With the bitcoin system, you can't "print more bitcoins", the banks can't control the money supply.

The second factor relates to price,