An octosyllabic line is a line of verse with eight syllables. The syllables may follow any pattern of stress, so an octosyllabic line is not the same thing as iambic tetrameter, even though iambic tetrameter also often has eight syllables.
Writers use octosyllabic lines in many forms, from ballads to rhyming stanzas, because the length is short enough to feel quick and musical while still allowing a complete phrase or image.
Example
Moonlight gathers on the fence
while the garden holds its breath.
Silent moths in silver bends
circle close and drift away.