This is a great question. There are many things that can be stated in connection to this. So, let me give you three points.
First, to invest in precious metals (gold, silver, platinum, palladium) means that a person puts in money to buy precious metals with the hope of future gains. In other words, people invest in precious metals, because they believe that price of metals will rise in the future.
Second, most of those who invest in these precious metals also believe that there will be inflation. So, metals are an inflation hedge. As you know the governments of the world have been printing a lot of money.
Third, there are several ways in which to invest in metals. You can actually buy bars or coins, or buy these metals via stocks on a stock exchange.