I think you mean the URL in the ledger creation command, e.g., 'hyperledger ledger register newcoin https://newcoin.com'. This would just be the official site of the asset or company, like bitcoin.org is for bitcoin. Currently, this is just used so people can get extra information, but it could be used in the future to remove spam currencies if there is no official site (although we would not be able to do this ourselves, it would require the consent of the entire consensus pool).