It represents off board artillery support. It is a tactic card that does not actually activate any of your artillery units.
All you do is pick a single target, anywhere on the board, and roll the 4 dice. There are no reductions for terrain and flags can't be ignored.
That sounds like a good way to explain it.
I don't have the card in front of me, but I think you hit on the unit's symbol, grenades, and maybe stars?
The card reads, "Target any 1 enemy unit. Roll 4 battle dice, ignoring any terrain battle die reduction. Score 1 hit for each die matching the unit's symbol or grenade. For each flag, retreat 1 hex. Flags may not be ignored."