The line integral over a scalar field f can be thought of as the area under the curve C along a surface z = f(x,y), described by the field. 

Line integral of a vector field, F. The particle (in red) travels from point a to point b along a curve C in a vector field F. Shown below, on the dial at right, is the field's vectors from the perspective of the particle. As it changes orientation, the axis arrows rotate to illustrate the changes in reference. The blue arrow is the field vector relative to the current orientation of the particle. The dot product of the tangent velocity vector (in red) and the field vector (in blue) results in the value represented as a green bar. This bar "sweeps" an area as the particle travels along the path. This area is equivalent to the line integral.