A packer is a prosthetic of a flaccid phallus or penis - meant to be worn as a way to create the look and feel of a bulge for gender presentation purposes; it usually looks like an anatomical penis. A dildo is an erect phallus designed for penetrative play - these come in all shapes and sizes, including realistic dildos (also called representational dildos) that look like a penis.
There are some dildos that double as packers, these are called “pack and play” dildos, and are able to be posed “flaccid” and worn under clothes or posed erect to use for penetrative play.