However, I don't know what a line pair is.
What are you guys saying in non-scientific terms?
Like Eric says above, the whole goal of MTF testing is measure how well a lens (optical system) can copy a known pattern. The pattern and the way it is measured is intended to be representative of how real world scenes are rendered (even though they look nothing alike), and the MTF is not a complete description of a lens' performance (factors such as bokeh, curvature of field, close-focus performance and other issues are not captured by the standard single MTF curve).
Still it is a useful tool in measuring lens ability to render fine contrast (what we'd call 'detail' in an image) and coarse contrast (what we'd call microcontrast in an image).
The fine detail is represented a chart with typically 30 or 40 line pairs per millimeter (40lp/mm or 40 cycles/mm). Sometimes you'll see this written as 40 lines per millimeter, but this is incorrect--they mean 40 line pairs per mm. This means that you will ask the lens to accurately reproduce a line that is 1/80mm in width, then reproduce a strongly contrasting line immediately adjacent to it, also 1/80mm in width, again and again, repeating across the frame. This is quite difficult, and even a very good lens might only reproduce around half or even less of the available contrast of the test target.
This level of performance would be shown by a line travelling across the the middle of the graph (representing 50% performance). In practice, the lines are never perfectly straight, as it is progressively more difficult to achieve the same level of performance as you move away from the center of the lens' image circle. Lines will typically curve down, indicating poorer reproduction.
The microcontrast is represented by the 'coarse' lines, at perhaps 10-15lp/mm. Because these lines are wider, they're somewhat easier to reproduce (these lines are always higher on an MTF chart).
Hope that helps,