When the average kinetic energy of the molecules goes up (a rise in temperature), the average speed of the molecules increases. And lower average kinetic energy of the molecules means they have lower speed. However, a change in average kinetic energy is not directly proportional to a change in average speed. 1.
An appreciation means an increase in the value of a currency. It means a currency is worth more in terms of foreign currency.
Bitcoins have value because they are useful and because they are scarce. As they are accepted by more merchants, their value will stabilize. See the list of Bitcoin-accepting sites. When we say that a currency is backed up by gold, we mean that there's a promise in place that you can exchange the currency for gold.