I was intrigued because I didn't know what it meant.
I Googled it when I got home and here's an excerpt of what I found:
Free Derry (Irish: SaorDhoire) was a self-declared autonomous nationalist area of Derry, Northern Ireland, between 1969 and 1972. Its name was taken from a sign painted on a gable wall in the Bogside in January 1969 which read, “You are now entering Free Derry". The area, which included the Bogside and Creggan neighbourhoods, was secured by community activists for the first time on 5 January 1969 following an incursion into the Bogside by members of the police force, the Royal Ulster Constabulary (RUC). Residents built barricades and carried clubs and similar arms to prevent the RUC from entering. After six days the residents took down the barricades and police patrols resumed, but tensions remained high over the following months.The Free Derry era is commemorated by the Free Derry wall (see below), the murals of the Bogside Artists and the Museum of Free Derry.
You can read more about it here.