In cycling, a "pack" of riders refers to a group of cyclists who are very close together.
In Flamme Rouge, a pack is any group of riders without an empty space between them. They can either be side be side in the same square, or one in front of the other on the same side of adjacent squares (eg both in the left lane). A pack may have many riders spread across several squares.
Additionally, a single rider is also considered a pack of one.