This DropĀ-seq tool extracts reads per cell barcode (=the number of reads for XC tag in a BAM file). Based on these, a cumulative distribution plot is drawn. This plot can be used to estimate how many cells you want to extract from your BAM in the DGE matrix: the "knee" of the distribution shows this. The number of STAMPs are the number of cell barcodes to the left of the inflection point (the "knee"); to the right of the inflection point are the empty beads that have only been exposed to ambient RNA.
For more details, please check the Drop-seq manual.