Advisory is a broad category of services, similar to consulting. These advisory services are normally hand-tailored to the client's needs and wants, and are not so much a prepackaged service.
Some examples of advisory are:
Teaching a new bookkeeper how to do bookkeeping.
Reviewing the work of the client or on-staff accountants when they self-serve bookkeeping and accounting services and offering correction.
Building General Ledger Accounts
Consulting on accounting systems and processes
Fraud Investigations
And So Forth.