Final answer:
A sign with a red circle and a diagonal mark means 'do not do' the action shown, such as no entry or no turning, and is part of universal traffic signage.
Step-by-step explanation:
When you see a sign with a red circle and a diagonal mark across a black arrow or symbol, it means do not do whatever is shown. This type of sign is commonly used in traffic regulation to indicate that certain actions are prohibited, such as no entry, no turning, or no stopping. Understanding these signs is crucial for safe driving and adhering to traffic laws. Road signs are designed to be universal symbols that quickly communicate important information to drivers, helping to manage traffic and prevent accidents.