Tunneling is a shortened form of the term "tunnel vision," and refers to a sniper unintentionally becoming focused on a particular character or mission. This is generally considered a bad thing for the sniper to do: paying too much attention to one thing makes it much more likely that they'll miss other things the spy may be doing. On the other hand, a spy subject to tunneling can be caught much more easily when performing actions.
A sniper may intentionally choose to focus exclusively on one or two specific missions, especially if they know that the spy is obliged to perform them. This is typically referred to as camping and can be a viable strategy.