You buy them when the price is low and sell them when it goes higher than you bought it . This way you make a profit. (and you loose money if the price goes down but you can hold it and wait until it recovers)
For example if you buy a coin that costs 1$ and invest 100$=100 coins and the price of the coin rise to 1.2$ and decide to sell yours you will gain 20% profit 100 coins=120$ .
Hope i helped.