The Standby Phase occurs immediately after the Draw Phase. Nothing is required of either player during this Phase unless a Card mentions it.
Effects of some Cards, such as "Armed Dragon LV3", occur during this Phase.
Many Maintenance Costs Cards, such as those of "Messenger of Peace", are paid during this Phase. If multiple actions are required during the Standby Phase, the turn player chooses the order in which these actions are performed.
There are some Cards that can only be activated in the Standby Phase, such as "Curse of Fiend".
Trap Cards, Quick-Play Spell Cards, and Monster Quick Effects may be activated in this Phase.