The verb form of the word "buckle" means to bend, fold or give way under pressure. It can also mean to collapse or fail due to stress or force. In a more general sense, it refers to any instance where something gives in or breaks under strain. For example, if you were to say that someone's knees buckled when they heard bad news, it would imply that the person physically collapsed as a result of being emotionally overwhelmed.
In its noun form, "buckle" refers to any type of fastener or clasp used for holding things together such as belts and shoes. It can also refer to a piece of equipment designed to secure something in place like the seatbelt in an automobile.