Something that's agreeable is pleasant or gratifying. An agreeable travel companion is someone you get along with and whose company you enjoy.
An agreeable day might be sunny and warm, with a gentle breeze, and an agreeable meal is eaten with friends and tastes great. You can also use agreeable to mean "willing to agree," as when your sister is agreeable about loaning you her car for the weekend. In Old French, it was agreable, from agreer, "to please," or "to receive with favor," from the phrase a gré, "favorably," or "to one's liking."