The backbone takes up the majority of your plasmid. It carries all the genetic information for your GMO that isn’t directly related to the gene you’re expressing. This includes resistance to antibiotics and the replication origin which controls the number of your plasmid present in any one cell at a time.
The backbone you’ll be using is called pSB1AC3 and is the area shown in red in the map plasmid below. pMB1 is the replication origin and has a relatively high copy number of 100-300 per cell.
You can find more information on ampicillin here and more information on chloramphenciol here.