When someone does a runner, he leaves a place in a hurry in order to avoid paying for something (like in a restaurant) or flees a difficult situation to escape punishment.
It also means to flee or quickly leave a place, usually so as to get out of paying for something or to avoid trouble or the law.
Like many British idioms, this particular idiom originates from one of Shakespeare’s popular plays, Anthony and Cleopatra.
Example
> At this point, the con artist did a runner with all her money.
> The teenagers did a runner as soon as the cab stopped, leaving the driver to foot the bill.
> The young man, unintelligible with drink, did a runner when the police went up to question him.