The aim of the grandfather, father, son or GFS backup strategy is to allow the data to be restored back to virtually any previous state.
For exampe the grandfather set would commonly contain 12 tapes (or disks or CDs or whatever). These are complete backups made at the end of each month. The father set would contain 4 tapes, made at the end of each week. These could be full or partial backups for the week, depends on the data. The son set contains 4 tapes, one for each day of the week, except Friday when the father backup is made (assuming a 5 day working week) these could be partial backups for the day or full backups.
If partial backups are used then if the data needs to be restored you first restore the most recent granfather, then father then son.
Think about what happens if restoration is needed but one or more of the backups are stuffed.
Hope this helps.