The three headings are just how Annual Leave is categorised.  To understand them you first must remember that an employees Annual Leave year runs for a 12 month basis annually from when they started.  At the end of each year the balances are moved from one area to another as follows:

The leave that you accrue during the year (within your current holiday pay year) is called Accrued Leave and every pay this keeps increasing based either on hours worked or days paid (depending upon setup) so that at the end of your 12 months you will have accrued your total entitlement, say 20 days or 160 hours.

At the end of your holiday pay year the total Accrued Leave is transferred to Outstanding Leave and your accrual starts all over again.  So on the date of your anniversary Outstanding would show 20 days and Accrued would be zero.

If during the year you take leave (and you have no days left in Outstanding as you have taken them all) then the leave you take is called Advanced as it is effectively an advance of your current years entitlement as strictly speaking you are not entitled to your leave until you have completed each 12 month period.

The system takes leave from your Outstanding balance first and then from Accrued so you cannot have a situation where there is a figure in Outstanding and Advanced.