This is a common term used for teaching children how to stay safe when away from parents, the term makes children aware of the danger of unknown people. Children need to understand how to stay safe when they are not accompanied or are approached by someone they don't know; this is started from a young age in order to drill the message into their heads. Children are the most vulnerable so teaching them this message is crucial, they should be aware of this from a young age.
It is important to tell your child how to act and respond if this happens, this may be a scary or tough subject to ring up as you don't want to frighten them however it is vital that they know this information.