NFPA 704 is a standardized labelling system for chemicals. NFPA stands for National Fire Protection Association. The 704 part is the labelling standard.
The blue diamond on the left is the "Health" diamond. 0 (or blank) means no substantial health hazard. 4 means a great and direct health hazard.
The red diamond at the top is the "Flammability" diamond. 0 (or blank) means the substance is generally nonflammable. 4 indicates extreme autoignition / explosion hazard.
The yellow diamond on the right is the "Reactivity" diamond. 0 (or blank) means the substance is generally unreactive. 4 indicates an extremely reactive (caustic, acidic, oxidizing, reducing, etc) substance.
The white diamond at the bottom is the "Other" diamond. It is usually filled with a self-explanatory symbol or characters