Securityholder Reference Number (SRN) and is a unique identifier for your security holdings with a company.
You can find an SNR on the initial holding statement sent to you by the company in which you purchased shares or on the dividend payment advice. They most often start with an "I" and is followed by 10 or 11 digits e.g. I0007627234. You will have a different SRN for each company that you own shares in.
Share registries handle the everyday administration for ASX listed companies. If you are unable to locate your SRN, a quick call to the share registry of the company you hold shares in will be able to help you.