Price is demand and supply. When demand is high, it often drives the price higher and opposites when demand is low and supply is higher.

If you compare the price from multiple exchanges in real time of any coins. It is almost impossible that all of them will be trading at an exact same price.

There is always a gap in price between each exchange.

Some traders are using these advantages by buying a coin from exchange A and transfer to sell on exchange B.

I highly recommend you to read further about arbitrage trading which will give you a better understanding.