The Excel Indirect function converts a text string into a cell reference.
If you type the reference B1 in an Excel formula, Excel understands that this refers to cell B1. However, Excel does not understand the text string "B1" to be a reference. Therefore, if your cell reference takes the form of a text string, you will need to use the Indirect function to convert this into an actual cell reference.
The syntax of the Indirect function is:
where the arguments are:
ref_text    The text describing the reference. 
[a1]   
An optional logical argument that defines the style of the ref_text reference. This can be either:

By default, Excel uses the A1 referencing style. If you want to use the R1C1 referencing style, you will need to set this in your Excel options. To learn more about referencing styles, see the Excel Reference Styles page.
Column A of the following spreadsheet shows four simple examples of the Excel Indirect function.
Formulas:

Results:

Note that, in the above spreadsheet:
For further details and examples of the Excel Indirect Function, see the Microsoft Office website.
If you get an error from the Indirect function, this is likely to be the #REF! error:
#REF!    Occurs if the supplied ref_text is not recognised as a valid cell reference. 