To be released from a criminal charge without punishment, or not receive the expected or deserved punishment.
This phrase originated in the USA in the early 20th century. 'Walk free' had been used literally many times before that of course. In 1764, Roger L'Estrange published a collection of Seneca's essays under the title of Seneca's Morals by Way of Abstract, which included this line:
What (says he) Shall I live in Trouble and Danger myself, and the Contriver of my Death - walk free, and secure?
The meaning there isn't far removed from the current American 'walk away from a crime without any punishment'. It is doubtful that whoever began the 20th US usage consulted the essays of Seneca first, however. Our present use of the phrase comes from the USA, not ancient Rome.
The earliest modern usage of the phrase that I've found is from the Connecticut newspaper The Bridgeport Telegram, December 1925:
"She walked free today after her fourth arrest in as many months."
Over time, 'walk free' has been shortened - in the USA at least - to just 'walk'. The US authors Sara Harris and John Murtagh's book about prostitution Cast the First Stone, 1958, includes an early citation of 'walk' in that context:
"This is a good judge sitting today... He's liable to call you a tramp, but if he can, he'll let you walk."