Sum Index Field Technology(SIFT) is a built-in technology that exists in Dynamics NAV and is used for totaling.
In other ERP systems, totals, subtotals, and balances are calculated and stored somewhere. This calculation has to be redone over and over so that numbers are up-to-date.
In Dynamics NAV, as a developer you don't have to worry about that as it is done by the SIFT technology. Creating a new subtotals field is as easy as indicating in the field properties that the field is a
Flowfield and specifying the formula that is going to be used to calculate the subtotal. After this you will not have to worry about keeping it up-to-date.
As a user, you know that balances for your G/L accounts, customers, ...