To Interpret nested if statement in Excel

Can anyone help me to interpret the following nested if statement in Excel? Thanks.

"The Imputer", "detasian", and "detnphi" are worksheet names of an Excel workbook. cell C9 and cell C10 are user inputs, and cell c16 is a result from an intermediate calculation step.

"value of cell c17"=IF(C16="A",VLOOKUP('The Imputer'!C9&'The Imputer'!C10,detasian!O23:P40,2,FALSE),IF(C16="P",VLOOKUP('The Imputer'!C9&'The Imputer'!C10,detnhpi!L23:M40,2,FALSE),""))