Jamming is an attack in which radio waves disrupt Wi-Fi signals. Wi-Fi itself uses radio waves for communication and is therefore susceptible to being jammed by devices that broadcast noisy signals on the same frequency. These signals override any other wireless signals that a wireless receiver is attempting to pick up on. By jamming a Wi-Fi signal, you can trigger a denial of service (DoS) and disrupt the flow of communications.
Physical jamming devices can send these disruptive signals to wireless receivers in a targeted area. However, such devices are illegal in many jurisdictions, including the whole United States. You should consider the legality of radio jamming in your area before performing it as part of a test.
Although not exactly "jamming" in the sense of physically disrupting radio transmissions, some in the industry may use the term jamming to refer to a disruption of wireless communications that breaks the link between client and access point. This is called deauthentication, and you can use it to knock a client off a network and prevent it from sending and receiving communications. For example, wifijammer is a Python script that can disrupt all WAPs in an area, only to be constrained by the effectiveness of your wireless interface. You can also use wifijammer to perform more targeted attacks to disable only select Wi-Fi networks in an area, or even specific clients.