Three short lines, and almost nothing else: a speaker says there is a weight on their chest, and that it stops them from breathing without pain. The poem never says what the weight is, and that's what makes it linger.
Taken literally, it could describe a physical ache, like a tight chest or trouble getting a full breath. But most readers will hear something emotional too. Grief, anxiety, stress, or sadness are often described as heaviness pressing down on us, and the poem seems to invite that reading without naming a cause. That is an inference from the wording, not something the text confirms.
The mood is quiet, strained, and a little trapped. Breathing is automatic, so having it turn painful suggests that even the simplest part of living has become hard. The brevity helps. The poem itself feels short of breath, as though the speaker can only manage a few careful words at a time.