This sign is used to indicate to substances that could cause damage to the living creatures, mainly humans. These biohazards could include many forms of microorganisms, viruses, and toxins which originates from biological sources. The symbol is meant to be a warning that if you got exposed to a certain substance, you should be cautious.
This symbol was developed in 1966 by a chemicals company. The goal was to create a new symbol that is meaningless but catchy so they can use it to educate people about its meaning. When they were testing the symbol for memorability, it had a very high score.