I am reading about the precise measurement of top quark mass from: http://arxiv.org/abs/1510.04483
In the second page there is this statement
"The 68% and 95% CL contours for the indirect determination of mW and mtop from global SM red, dashed curves, respectively. For negative values of λ, the lifetime of the SM fits to electroweak precision data"
I am a noob in analysis and did not understand what CL contours mean.