"Soft targets" are any particular venue that is not well protected or fortified, making it particularly vulnerable to an attack. In the context of terrorism, "soft targets" can refer to areas where large amounts of people freely move and there is so much activity that ensuring controlled crowd flow is significantly more difficult than at an airport or government office.
Soft target is a military term referring to unarmored/undefended targets needing to be destroyed. For example, a soft target would be an automobile, a house, or assembly of people while a hard target could be a main battle tank or a well defended installation. (Soft Target, Wikipedia)
Examples of soft targets are shopping malls, movie theatres, outdoor concerts, hotels, bars, nightclubs, or sporting events. In these settings, the high number of people, the convergence of multiple events happening, free flowing traffic, and the open nature help to make the target a "soft target." These targets are particularly vulnerable to terrorist attacks because they are so difficult to control. The terrorist could blend in with the surroundings and be able to make an impact almost unnoticed:
After the Sept. 11 attacks, security was boosted for airports, government offices, and other high-profile "hard" targets as well as nuclear and chemical plants. And while local authorities were given technologies and resources to respond to an attack--more paramedics and mobile command centers--their ability to prevent attacks on soft targets remains limited--whether it's a crowded mall during the holiday shopping season or an outdoor summer concert. (Matt Vasilogambros, National Journal)
The soft target is defined by its unprotected status and by its open, exposed nature and, thus, its vulnerable and "soft" condition. The challenge is to keep the integrity of the function of the target while building up significant protection to reduce (if not eliminate) its "softness" while deterring the potential for harm:
You have to combine the ability to allow people to go in and out and do what they want to do on a normal basis, and separating those who are intent on doing bad things. (Vasilogambros, National Journal)
This idea of "normal" and elements that are part of everyday life further define the "soft target."