Initiative: When you roll for Initiative, you roll a D20 and add your Dexterity Modifier
Initiative Tie-Breakers. In the event of a tie, you add your Dexterity score/100 to your initiative. e.g. A character with a 15 Dexterity score that rolls a 10 on their initiative die would have a 12(10+2). On a Tie on initiative 12, this character would have a 12.15 for initiative.
Delaying Your Turn (Dropping Initiative): Before you take any action or movement on your turn, you may delay it. To do so you drop your iniitative. Detail to the DM where in the turn order you would like to delay your turn to. Once you do so, you cannot return to your original initiative position without delaying your turn to that original position in the turn order.
Use Cases: In some cases, a Dedicated Healer may roll high, and take their turn before their party. In such a case, the Healer may be limited in their action economy, and the Healer may choose to Delay their turn until their last party member goes, this would allow them to potentially Heal for their first turn.
Ready Action:
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the goblin steps next to me, I move away.”
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell’s magic requires concentration. If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.
Countering a Readied Action: In most cases, unless you have a reaction available, you are at the mercy of a readied action. In some cases, you DM may permit you to use your reaction to make a Dexterity Check. In such a case, you may be permitted to perform a single available Action or Bonus action that you have available, but not both, and use your reaction to trigger it. In such a case reactions are performed in initiative order, with your check taking the place of your initiative.
e.g. Phenix the Moon Druid dashes out of cover but is assailed by several guards that have readied their attacks to fire at the first creature to emerge fro mcover. The Guards' initiatives are 15,14,10, and 4. Phenix asks the DM if he could Wildshape upon seeing the guards. The DM allows him to make a Dexterity Check. Phenix subsequently rolls a 13. Two guards manage to get their attacks off before Phenix transforms, after which, the final two guards have their attacks triggered.