Interpreting the code "newcolumntype"

p is a standard latex array column specifier

begin{tabular}{cp{3cm}}

means that each cell in the second column is a parbox of width 3cm

Q here is defined as a new column type taking an argument Q{...} where #1 denotes the argument. so Q{3cm} expands to >{centeringarraybackslash}p{3cm} which is a p column with centering applied to each cell.

See the array package manual (texdoc array) for more details.