This is the 433rd most frequent Japanese word.
平
"平" means "flat," "calm," or "peace."
The word 平 (taira) is used here as an adjective root meaning "flat," describing the surface of the desk in this sentence.
この平らな机はとても使いやすいです。
This flat desk is very easy to use.
Here, 平 (hei) forms the noun 平和 (heiwa), meaning "peace," expressing a wish for a peaceful state of the world.
世界が平和でありますように。
May the world be at peace.
In this context, 平 (hei) is part of the word 平穏 (heion), which means "calm" or "peaceful," describing the nature of someone's personality.