恬 is a bound form (b.f.) meaning 'peaceful, quiet, serene' — it almost always appears as part of two-character words rather than standing alone. You'll encounter it in literary or formal contexts describing a calm, untroubled state of mind or atmosphere, such as 恬静 (tranquil), 恬淡 (indifferent to fame or gain), or 恬不知耻 (shameless — literally 'calmly unaware of shame'). Because it's a bound form, you cannot use 恬 by itself in a sentence; always attach it to another morpheme.