the little rhombus thing is an NFPA 704 “fire diamond”. each color signifies a danger: yellow is (chemical) reactivity, red is fire hazard, and blue health hazard. white is for other stuff.

the numbers are on a scale of 0-4, where zero is basically zero hazard and four is most dangerous. the W means “use no water”, as in mixing it with water can generate dangerous fumes, cause explosions, etc. OX is short for “oxidizing”.

tl;dr this sign means there is a ton of dangerous stuff in that room