height of; middle of; full bloom
Describes the peak or most active stage of a season, event, condition, or state. Often used with ～の真っ盛り or ～真っ盛りだ.
今は桜の真っ盛りです。
The cherry blossoms are in full bloom right now.
彼は今、仕事の真っ盛りで忙しい。
He is busy right now in the middle of work.