。
It's really hot today.
Literal
Today [topic-は] very hot.
A textbook weather sentence. 暑い is the i-adjective for 'hot weather' — distinct from 熱い ('hot to the touch'). Both read あつい. Watching out for these homophonic kanji distinctions is part of intermediate Japanese: 寒い (cold weather) vs 冷たい (cold to the touch), 暑い vs 熱い, etc.