You might want to have a chat with your advisor to understand these terms, what under-/overflow means, and how to plot the pull. Just quickly:

You measure y_m \pm \Delta y, the fit for the corresponding x value is at y_f. Then the pull p is p = \frac{y_m - y_f}{\Delta y}.

If you create a histogram `TH1F hist("h","h", 10, 0.45, 0.55)`

and call `hist.Fill(0.51); hist.Fill(7.3);`

then you will see only one entry in the histogram, as the second `Fill`

adds an entry out of range (“overflow”, as it’s larger than the upper limit of 0.55). You can see the number of under-/overflow entries with `gStyle->SetOptStat(111111)`

before drawing. IOn your case, about 12 entries ended up in the range of 0.45…0.55, the rest of the 200 entries were in under-/overflow bins.

Now - we’re happy to help with how to use ROOT, but we get to our limits with a general introduction to experimental particle physics You might find friendly people here in the forum who help you nonetheless, but it’s better to consult with your advisor / physics group / professor / … for these kind of non-ROOT-related questions.