Creatures can't attack you unless their controller pays for each creature they control that's attacking you.
Propaganda is one of the premier defensive enchantments in Commander, and its #119 rank is well-earned. In a four-player game, the "pay per attacker" tax compounds fast — swinging at you with three creatures costs an opponent , often more than their resources or their will allow. The result is that aggression simply gets redirected to easier targets.
This is a staple in any blue deck that wants time: combo decks assembling a kill, control decks grinding out value, superfriends/planeswalker shells protecting their loyalty, and slow stax builds. It pairs beautifully with Ghostly Prison, Sphere of Safety, and Norn's Annex to make attacking you near-impossible.
The catch: it only stops attacks aimed at you. It does nothing against your planeswalkers being attacked, burn, mill, or combo kills, and ramp-heavy decks can simply pay the tax. Skip it in fast, proactive decks that win before defense matters — you'd rather run interaction or threats. It's a control/midrange card, not an aggro one.
The payment is made during the declare attackers step at the same time you are declaring the attacker.
If there are multiple attacks in a turn, then you have to pay for each attack.
If there is more than one Propaganda on the battlefield, the cost is cumulative.
Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.