The height or peak of something (e.g., full bloom, peak season, height of activity).
桜が真っ盛りで、公園は花見客でいっぱいだ。
The cherry blossoms are in full bloom and the park is full of hanami visitors.
夏休みの真っ盛りには海水浴場が混雑する。
At the height of summer vacation, the beaches get crowded.
彼は仕事の真っ盛りで、なかなか休めない。
He's at the peak of his workload and can't get much rest.