A dcbb is a double crochet that is worked into the top loops of the stitch behind and 2 rounds below the next stitch, which is usually a special stitch.
Technically, this is a dc_0-2-b, double crochet behind the st 2 rounds below the next stitch, but we leave it as a dcbb.
In order to avoid unwanted increases, we usually skip a stitch behind each special stitch we make, for example a front post stitch or a stitch worked into the front loop of a stitch in a round further below (unless instructed otherwise). This leaves the top loops free, in which we can then work into in the following round.
The dcbb is useful to close gaps behind special stitches and to make a motif more dimensional, as it brings the front stitches forward. It can be used when working in rounds as well as with forwards-and-backwards rows.