Onomatopoeia imitating the sound of a cicada's 'cry'.

This is usually written as "miin min" (ミーン ミン) or "jii jii" (ジー ジー) in Japanese comics. Several other sounds also exist for different species of cicadas. 

Cicada cries are a staple of summer scenes and are frequently used to "convey" weather conditions that are both "hot" and "muggy".
cicada cry (sfx)