Lenningrad was never entirely encircled by Axis troops. The Soviet defenders managed to cling to a short section of the eastern shore of Lake Lagoda, the 17,700 square km lake east of the city:
The lake was used to force a thin trickle of supplies into the city using water craft in the summer and along an 'ice road' during the winter. The route was perilous, under attack by German aircraft and artillery, but was never cut by German troops or Finish forces from the north. The meagre supply line remained open for the duration of the siege.
In 1943 this 'road of life' is supplemented by the land corridor opened along the coast of the Lake by advances made during Operation Iskra. This allowed a far greater volume of relief into the city.
So at this point the blockade that had resulted in 1.4 million civilian and military deaths in the city (many from starvation) is broken, but the siege lines around the city remain until January 1944 when the besieging armies are finally pushed back.