When something is agreed, it's been talked over and found to be acceptable to everyone. If noon is the agreed club meeting time, it means all the members are aware of that plan.
The adjective agreed describes something people have reached an agreement about. The school board chair might say, "According to the last vote, it's agreed that we should allocate more funds to music programs." Or your friend could remind you that tomorrow's agreed plan is a trip to the mall. You can also use this word as an interjection, to emphasize your agreement. If your pal says, "So we'll go to the mall tomorrow?" You might respond, "Agreed!"