Something reassuring comforts or inspires you. A reassuring smile can help to calm a nervous speaker who's about to give a speech.
When you're anxious about singing in the talent show, some reassuring words from your best friend or your dad's reassuring hug can make you feel much more relaxed and confident. And fearful flyers sometimes find the calm voice of the captain reassuring. This adjective comes from the verb reassure, or "restore to confidence," which adds re-, or "again," to assure, which has the Latin root securus, "safe or secure."