A chemical substance is a stable arrangement of atoms. Almost everything you care about is made of chemicals: People, trees, computers, prescription-strength antihistamines, tacos — all that stuff.
Some chemicals, like cyanide, can kill you. Other chemicals, like water and sodium, keep you alive (but can also kill you), and still others don’t affect you much at all — at least not in the amounts you’re likely to find them.
I’m telling you this in case you are afraid of chemicals. Being afraid of chemicals is like being afraid of people. Some people would definitely harm you given the chance, but others would give you a big slice of chocolate cake or fix the leaky pipe under your sink.
Like people, chemicals are not necessarily bad for you. If you want to know whether a person or a chemical is dangerous, you’ll have to do a little detective work. Google it. Read some books. Ask a chemist.