State Code is a numerical code assigned to each state and union territory. The first two digits of GSTIN denotes the State Code of the state in which the taxpayer has obtained registration.
If the state code of seller is the same as state code of place of supply, CGST and SGST are charged. If the state code of seller is different than state code of place of supply, IGST is charged.
You can refer to the list of State Codes here - List of State Codes in India