You can find this info at URL:
http://root.cern.ch/root/html/TH1.html#TH1:PainStat
In short, this URL says:
// The type of information printed in the histogram statistics box
// can be selected via gStyle->SetOptStat(mode).
// The parameter mode can be = nemruo (default = 111100)
// n = 1; name of histogram is printed
// e = 1; number of entries printed
// m = 1; mean value printed
// r = 1; rms printed
// u = 1; number of underflows printed
// o = 1; number of overflows printed
// Example: gStyle->SetOptStat(11);
// print only name of histogram and number of entries.
Rene Brun