Emission Curve or Emission Schedule defines the way of how new coins will be released and how it's gonna happen. Coins can be mined, released at once or in smaller parts.
The emission curve can help us assess the value of the coin/token and predict it's volatility. For example, mined coins with stable supply that's decreasing over time as the mining difficulty goes up, should be on the more stable side. While coins/tokens dumped onto the market in large chunks, will experience more frequent ups and downs.