A Resource ID, or an RID of the spreadsheet is an alphanumeric string that forms part of the spreadsheet's URL. The RID always follows the 'open/' part of the URL.

For instance, in this URL, 

https://sheet.zoho.com/sheet/open/hf6ye53f07d5c155a4639b7b8e26b1ab4a025/sheets/Sheet1/ranges/A1,

the RID would be: ​hf6ye53f07d5c155a4639b7b8e26b1ab4a025