Single shot is the basic attack available with most firearms in Fallout, Fallout 2, and Fallout Tactics. As the name suggests, it uses a single unit of ammunition.
It's the cheapest attack mode for weapons that feature it and targets the torso by default. Any critical hits will be rolled on the torso critical hit table. Characters with Fast Shot can only use this fire mode - or burst fire - limiting them to hitting the torso by default.