The line of action is the line along which the force act. The moment arm is the shortest distance between the line of action and the pivot.
There is then a general formula for calculating torque and two very useful shorcuts.
As we will see later, torque is really a vector. If we restrict to rotation in a plane then the vector nature of torque just appears in the fact that torque has a sign. A torque that wants to make an object turn CCW will be positive, while CW will be negative.
Practice calculating torque by doing example 12.8 and 12.9 without looking at the solutions. These should be not be too hard.