Canonical plosives, such as [b], always involve the velum being raised to seal off the nasal cavity. This velic closure means that the air coming up through the vocal tract from the lungs can't leave through the nose and is forced, instead, into the oral cavity.
A second feature of such plosives is a complete stricture/blockage at the notional place of articulation which prevents the air from leaving through the mouth. The air coming up from the lungs compresses behind this blockage as the lungs continue to try to push air upwards into the vocal tract. Notice that the increased intra-oral pressure is an essential feature of the production of the plosive, and that this would not be possible if the velum was not raised, because the air would simply escape through the nasal cavity.
With a canonical plosive, the articulators forming the blockage suddenly come apart resulting in audible plosion, and the compressed air behind the blockage rushes cleanly out of the the oral cavity.
However, that last stage only applies to a canonical plosive, for example a canonical [b]. There are several other ways that the compressed air may leave the system depending on the nature of the following segment. One of several alternative ways for the compressed air to be released is for the articulators to maintain the blockage at the place of articulation, but for the velum to suddenly lower causing the compressed air to leave via the nasal cavity. In this case, the release will result in audible nasal plosion. Such a release is traditionally called a nasal release.
In terms of the narrow transcription of a plosive with a nasal release, this is normally indicated by a small superscript < n > to the right of the consonant concerned: [bn]