To watch a 3D movie, you need 3D glasses that basically create the illusion of a three dimensional picture. Passive 3D glasses are one of the two main types of glasses. Passive 3D works by using polarized lenses that block out certain wavelengths of light to each eye, creating the illusion of depth and enabling a 3D look to a movie. Movie theaters showing a 3D movie provide passive 3D glasses to their customers. Many 3D TV manufacturers also include passive 3D glasses with the purchase of their 3D TVs.
There are also Active 3D glasses, which work by use of small LCD screens that alternate the dimming of the lenses. The TV itself controls the dimming by sending an infrared signal to the glasses, telling it which of the lenses to dim and when. Many people prefer Passive 3D to Active 3D due to the cost and weight of the glasses. Passive 3D glasses are much cheaper and lighter than Active 3D glasses. Also, while it's a matter of opinion and dependent on the TV to an extent, some people consider Passive 3D to produce a better image quality versus Active 3D.