Ready action

As explained in PHB/DMG

• There is no "Hold turn" action. On your turn you can prepare to do a thing as a reaction. Your turn is spent doing the preparation.
• You can Ready an Action if you don’t know what to do or need to wait for an ally or monster to do something first. You can only ready one thing (action or move) (house rule, you can ready your movement as part of your action, but it must be pre-defined where you are moving) and you must declare a specific trigger that determines when you can do that thing.
• You use your reaction to for your Readied Action. So you cannot also take an opportunity attack while holding a Readied action. If you do take an opportunity attack, you forfeit your readied action.
• An important note for spellcasters, Readying a spell actually casts the spell normally as your action and then you are holding the magic to release it as a reaction. If the trigger was never met to release the spell, the spell slot is still consumed. This is why spellcasters generally only ready cantrips. Also note that readying a spell requires concentration so you can't concentrate on another spell while you ready any spell.
• With readied spells, because they are cast normally as part of the action, they can be Counterspelled only when they are cast as part of the action, but not when they are released as the reaction.
• If you’re readying an action and the trigger hasn’t happened yet, you can choose to take an opportunity attack if something provokes one, but doing so counts as your reaction and the readied action is forfeit (and spell slot used if it was prepared.)
• You can only ready 1 action. You cannot ready an alternate action. You cannot for example say "I'll do this if X happens but I'll do Y if it does not." You can only prepare 1 action.