Some metals are very unreactive. This means they do not easily take part in chemical reactions. For example, platinum does not react with oxygen in the air, even if it is heated in a Bunsen burner flame.
Some metals are very reactive. They easily take part in chemical reactions to make new substances.
Other metals may be more reactive than magnesium, or in between magnesium and platinum. If we put the metals in order of their reactivity, from the most reactive down to the least reactive, we get a list called the reactivity series.
If you want to learn the reactivity series, you could try making up a mnemonic or silly sentence to help. Here's one:
"Pond slime can make a zoo interesting - the long crinkly sort goes purple."