Well, first you need to have a Tyranitar ex from Dragon Frontiers to use the Shockwave Marker or a Gardevoir ex (also from DF) to use the Imprison Marker.
The attacks and powers on these cards explain how to use their corressponding marker, but basically each of the Pokemon can place an effect that will stay even on the bench. That's why the marker is needed. Multiple Pokemon could have one of these effects on them, over several turns. The markers are needed to keep track of it.
Once Gardevoir ex places an Imprison Marker on a Pokemon, that Pokemon can no longer use it's Poke-Power until the marker is removed. Currently, the only ways to remove it is by evolving, leaving play, or when a Tropius d is played.
Similar with Tyranitar ex, except the Shockwave marker in and of itself does nothing. But it allows TTar to follow up with a second attack that will automatically KO a Pokemon that has the marker on it.
Hope that helps.
By the way, if your son is not playing with either of those Pokemon, feel free to use them as Burn or Poison markers. Or trade them to people who didn't get a DF starter deck. They were only available in those.
