One type of physical hazard is fire.  If you see this symbol on a chemical container, it means the product can each on fire and burn easily.   Therefore, it is very important to avoid sources of ignition such as sparks and open flames when working with them.

Materials which present fire hazards can be classified according to how easily they can catch fire:

Flammables have a flashpoint under 100 degrees Fahrenheit, which means that at or below 100 degrees, the material gives off enough vapor to catch fire if an ignition source is present.

Combustibles have a flash points above 100 degrees Fahrenheit.  These materials must be heated to above 100 degrees before enough vapor is emitted to ignite.

Pyrophoric materials can ignite by themselves, without an ignition source, at temperatures below 130 degrees Fahrenheit.