Angela's only comfort, as she would say herself, were her Woodbine cigarettes, which she smoked with her husband (when he was home) while they sat by the fire looking at the firewood consume itself until it turned into ashes. That would be one of the symbols of the ashes: The sole comfort in Angela's weary and tired , and sacrificed life. Also, in the end of the book 'Tis which is the continuation of the story of Frank McCourt, he mention's how Angela's Ashes (after her death) will basically be inmortal for she had given Frank the opportunity to live the life he lived.
So, concisely, Angela was literally "burnt" by her husband, by her limitations, and by the want of a better life for her children. She literally got to a point where she thought she might as well give it all up. She saw a few of her children die, she suffered through poverty and hunger, she was in an unstable marriage, she suffered degradation, and she had to fight through it all as best as she could. When Frank finally made it somewhat in life, he noticed how the wear and tear of years had left a huge burnt soul out of Angela. That could also be the meaning of the word "ashes". - What was left of her.