How can acids be harmful?
They can damage your skin and other materials by corrosion.
Acids can be very dangerous, especially acids with a high concentration of H+ ions. H+ ions are very dangerous. Never swallow acids, as they can corrode (essentially destroy) your organs. This would obviously result in organ damage, and more severely death.
Typical safety precautions with acids include wearing safety goggles. This stops acids getting in your eyes so they don't damage your retinas causing blindness.
Also use tongs when handling hot test tubes of acid to avoid spilling it over you.