[1] If two substances can mix together and not undergo a chemical reaction, they are considered compatible.
Incompatible chemicals react with each other, and can cause corrosion, mechanical weakening, evolution of gas, fire, or other undesirable interactions.
Chemical compatibility is important when choosing materials for chemical storage or reactions, so that the vessel and other apparatus will not be damaged by its contents.
Many companies publish chemical resistance charts.
For example, bleach and ammonia, both commonly used as cleaners, can undergo a dangerous chemical reaction when combined with each other, producing poisonous fumes.