The British traffic sign showing a red circle around two cars, one black and one red, means “No overtaking.”
The sign prohibits drivers from overtaking motor vehicles on the section of road that follows it. The red car is conventionally shown beside the black car to represent the prohibited passing maneuver. The restriction continues until a sign indicates that the overtaking prohibition has ended or another rule supersedes it.
This is not the same as a no-vehicles sign, which prohibits traffic generally, or a two-way-traffic warning, which alerts drivers that traffic travels in both directions. The sign also does not necessarily ban every crossing of the centre line; specific exceptions and road markings can affect how the rule applies.
British signs use red circles for prohibitions and triangular signs for warnings. The familiar two-car design is therefore read as a restriction on passing, while a separate sign is used to mark the end of that restriction.