Google spreadsheets lets you reference another sheet from the spreadsheet that you're currently editing by using the ImportRange function. ImportRange lets you pull one or more cell values from one sheet into another.
Spreadsheet-key is a STRING which is the key value from the spreadsheet URL.
Range is a STRING representing the range of cells you want to import, optionally including the sheet name (defaults to first sheet). You can also use a range name if you prefer.
Given that the two arguments are STRINGs, you need to enclose them in quotes or refer to cells which have string values in them.
"abcd123abcd123" is the value in the "key=" attribute on the URL of the target spreadsheet and "sheet1!A1:C10" is the range which is desired to be imported.
Cell A1 contains the string ABCD123ABCD123 and cell B1 contains sheet1!A1:C10
Note: In order to use ImportRange, you need to have been added as a viewer or collaborator to the spreadsheet from which ImportRange is pulling the data. Otherwise, you'll get this error: "#REF! error: The requested spreadsheet key, sheet title, or cell range was not found."