Final answer:
The round yellow warning sign with an 'X' symbol and the letters 'RR' indicates the presence of radioactive materials and the need for special safety precautions.
Step-by-step explanation:
The round yellow warning sign with an 'X' symbol and the letters 'RR' indicates the presence of radioactive materials. This sign is commonly seen in places like nuclear medicine departments of hospitals. It serves as a warning to inform people that special safety precautions need to be taken when handling these materials.
Radioactive materials are used for the diagnosis and treatment of many diseases. Although they can be dangerous, they are handled by specially trained individuals who follow strict safety protocols to minimize risks. It is important to respect and take necessary precautions when dealing with radioactive materials.