When MARS has found the Best Model it will show you a screen like this:
MARS produces several summaries and reports and this display lets you choose which to look at. This panel tells us that the final model uses 8 of the 13 available predictors which MARS converts into 11 predictor terms. The model performance is better than you can get with a linear regression with a naive R-squared of 0.862. The GCV R-squared is the MARS estimate of how well this model would perform on new data. The GCV measures are always more conservative and more trustworthy than the naive measures.