A jar is a smooth-sided, round container that's often made of glass. Jars are useful for storing food, from rice to tea bags to cookies.
A wide-mouth jar made of pottery might hold Oreos, and a small, short jar could contain homemade jam. When you put something in a jar, you jar it. Another way to use the verb version of jar is to mean "shock painfully or unpleasantly:" "Every bump we drove over in the old Jeep would jar my whole body." The jar that's a container probably stems from the Middle French jarre, "liquid measure."