。
The sun sinks crimson.
Literal
Sun [subject-が] crimson-[adverbial-と] sinks.
あかあかと is a mimetic/adjectival adverb evoking a vivid red/bright color — derived from 赤い ('red') with the repeated form adding intensity. The adverbial と here marks this as a descriptive manner modifying 沈む. A compact evocative line.