A blockout is a void deliberately formed into a precast piece so it is not solid concrete where an opening needs to be. A boxout or insert is placed in the mold before the pour, and once the piece is stripped, that space is left open for a pipe, a conduit, or a connection.
On a manhole or box culvert, the pipe openings are blockouts placed at the right invert and angle. Getting them located correctly on the shop drawing is what lets the piece connect to the rest of the system in the field.