Suppose that there are N patients indexed by n = 1, 2, ..., N. Suppose that I estimate two different survival models M1 and M2. Model $i$ gives a risk score $r_n^i$ for patient $n$.
According to https://stats.stackexchange.com/a/49054/22409:
The index of concordance is a "global" index for validating the predictive ability of a survival model. It is the fraction of pairs in your data, where the observation with the higher survival time has the higher probability of survival predicted by your model. As far as I remember it it equivalent to a rank correlation.
Can I use the C-index to say that one model is better than the other? For example, suppose M1 has a C-index of 0.8, and M2 has a C-index of 0.7, does that mean that model M1 is a better predictor of patient survival?