message
If the match doesn't like what you just did, it will respond with a short message near were you tried to do something. For example if the game expects you to chose an attacker for the blocker you just declared, and you instead of clicking on an attacker click on a land - a hovering message will appear - like:
message
If you did something that might for example cancle an action, the game gives you back a warning message, big red letters in the middle of the screen:
Warning!
If the game awaits some feedback of you (and the card designer had some heart for the player), you'll get a notice what the card actually expects of you. This is a small message directly send from the card script to the "player".
Example Fire Dragon:
Helper!
If the background of such helpers is more "reddish" the card expects some input from the player. If the background is more "bluish" the card just wants to give you some information about what was happening.
See information panel and status panel.