Heart symbol in the Traffic Life Series that represents how many lives a character has left. Comes in three varieties: green, yellow, and red.
May be depicted in different ways by different artists and depending on the season: Limited Life SMP art often depicts it as part of a pocket watch, while in general many artists often depict it as a tattoo on the body or as a literal life meter.
It is also commonly used in a full red-yellow-green "traffic light" design to represent the series as a whole.