The speaker begins with a simple confession: she bites her nails when she feels nervous, angry, or scared. When she tells her grandmother, the advice is to paint them, because "pretty girls don't have ugly nails." The poem grows out of that small, careless sentence.
The speaker sees that polish only covers the problem. Red paint matches her bleeding nail beds, and she compares herself to someone scratching at the lid of a coffin, desperate for air. She imagines her own funeral, where everyone praises the pretty girl and never mentions the pain. The same hiding happens elsewhere: a smile covers a bruised heart, and prettiness buries broken self-esteem.
The mood is bleak and suffocating, and the repeated "scratching" makes it feel urgent and trapped. The ending offers no relief. She keeps scratching until her nails are red again, which suggests the cycle goes on.