"On initiative count 20..." What does that mean?

[Karsalem]

On talking about lair actions of a high level monster I run across this wording:

"On initiative count 20 (losing initiative ties), *Insert nasty boss mob here* uses its lair action to cause one of the following effects: "

I'm not sure as a DM how to use this.

[EvanescentHero]

Say you have a monster with an initiative of nineteen, and a player with twenty-one. The lair action occurs between those two turns. "Losing initiative ties" means that if someone has an initiative of twenty, they go before the lair action occurs.

[DireSickFish]

Treat it as if you had gotten a 20 on initiative. So if PC 1 gets a 22 Initiative, PC2 gets a 20, PC3 Gets a 16 and Lair Monster Gets a 3 The Initiative would look like this

PC1
PC2
Lair Action
PC3
Lair Monster

So the nasty actions happen every round but are independent of when the monster acts.

[Silavor]

After everyone has rolled for initiative, you add The Lair to the list as if it had rolled a 20. If anything else (a player, a monster, a companion, anything) also rolled an initiative score of 20, they get to go first before the Lair Action.

Every time The Lair comes up on the Initiative Order, your boss monster can use one of its Lair Actions.

[Karsalem]

Got it, thanks folks.