Cryptocurrency exchanges are simply online platforms that allow you to make accounts, and login to buy, sell, and trade cryptocurrencies. But, the price of a currency in one exchange might be different to that in another, and the prices of all the currencies are also always changing.
By tracking these numbers you can make a profit: You might buy low on one exchange and sell high on another. You could also buy one cryptocurrency and then invest into another that has a lower dollar per coin value.
This way you’d own more coins overall with the hope of the higher number of coins making you a profit compared to if you had stayed with the other coins, and owned less of them.