"Security theater" is a disparaging term for security measures, such as full body scanning, pat-downs, and random searches, that are meant to provide an illusion of security, even if the measures are not effective at providing security. The enhanced security measures were a result of the September 11th Attacks, although many of the measures, and the criticism of them, stems from after the Christmas "Underwear Bomber" incident.
People who have much more of a background and an interest in security, as well as people who have much more of an interest in civil liberties, have probably already argued ad nauseum about whether "Security Theater" is a necessary way to defend ourselves in an age of terror, or whether it is merely a way to give law enforcement agencies more power over our activities while also looking impressive. However, given some of the seemingly arbitrary nature of the measures, the idea that security theater is, indeed, theater, does seem to be warranted.
I have an additional belief that some of "security theater", while seeming to be a hindrance and an inconvenience to travellers, is also a covert selling point. The presentation of travel (and mostly air travel, although security theater is present in other places) as a high risk activity where normal activities and accessories must be scrutinized and examined is a way to show the traveller that they are undergoing an activity that is outside and above normal activities, a prestigious activity that is well worse the (steep) cost of admission. However, that is speculation on my part, and people might see it as far-fetched.