The verb allay is used when you want to make something better or eliminate fears and concerns.
When you allay something, you are calming it or reducing difficulties. It is used commonly in the context of to allay concerns and to comfort and some of its many synonyms are alleviate, decrease, mitigate, assuage and mollify. Allay comes from the Old English word alecgan, which means "to put down," as in literally "to lighten." So, if you can allay someone's fears, you are lightening their mood!