So a biohazard is anything that could damage the health of a living organism, although usually the term is used to refer to anything that could damage human health.
As you can tell – this will cover a lot of things. This means that different biohazards will harm us in different ways. There are some chemicals that can harm us by causing cancer. There are some chemicals (like acids) that might burn us if they get onto our skin.
Labelling something a biohazard is a good way to warn everyone using it that it is potentially dangerous and they should take care.