na-adjective
perfectly flat; completely level
Emphasizes that a surface is entirely flat with no bumps or unevenness. Often used with な to modify nouns.
この道は真っ平らで歩きやすい。
This road is perfectly flat and easy to walk on.
真っ平らな地面にテントを張った。
We pitched the tent on completely level ground.
平ら means 'flat' or 'level' in general, while 真っ平ら is an emphatic form meaning 'perfectly flat' or 'completely level'.
Formed by attaching the intensifying prefix 真っ (ma') to 平ら (taira, 'flat'). The prefix 真っ is a variant of 真 (ma) used before certain consonants for emphasis.