Having been "recalled to life" after being locked up in a hole of a French prison, Dr. Manette has not completely been able to reclaim his former respectable position as a physician. Instead, he has had a complete and utter mental break down and has taken up the hobby of cobbling shoes, making shoes from scratch. When Lucie and Mr. Lorry first enter the garret story, their eyes immediately discern:
"A white-haired man [who] sat on a low bench, stooping forward and very busy, making shoes" (I.2)
Clearly, Dr. Manette still struggles with the hardship of his internment; only through Lucie's love and generous affection is the doctor able to reclaim his sharp wit and be one who has truly been "recalled to life."
Dr Manette had been a prisoner in the notorious Bastille prison in Paris, France, for around 18 years. This long term of imprisonment has quite adversely affected him, especially his mindset, due to the tortures and hardships that he had to face.
After the French Revolution of 1789 , in July, when the revolutionaries captured the Bastille and set free the prisoners inside it (mostly falsely imprisoned by the old corrupt feudal aristocrats' orders), Dr Manette was also set free and found and ttaken home by Defage and his wife, who had once served the doctor before his incarceration. They take him home and keep him locked up so that he wont wander off, until Mr Lorry and Lucie Manete, the doctor's daughter, arrive to see him.
Of course, Dr Manette does not initially recognise Lucie-- and he is, in any case, totally engaged in mending or cobbling shoes-- an effect of his psychosis, or mental state, after all that he had to bar for so long. This activity gives him 'peace' and it is for quite some time that he goes on practicing it-- it is finally after a considerable time and effort that Lucie, by her love and daughterly affection, is able to win and 'normalise' her father, and wean him away from his hobby/pastime of mending shoes.
In a psychological sense, we can also see that this task is indeed a relief for Manette--a symbolic one, too, for there is a play of words inherent in the task: when he is cobbling/mending shoes, he is actually 'mending soles' i.e. mending souls -- which implies that his soul, his inner self, needs 'mending' too, after long years of angguish and pain and misery.