I've seen this in several monsters statblocks, but what does it mean? What is initiative count 20?

When your players roll for initiative, and you set the initiative order, set anything that says "on intitiative count 20" as if it had rolled a 20 on initiative (no modifier).  so, If Grok rolled a 12 (with modifier), Cyan rolled a 21 (with modifier), and Billy rolled a 19 (with modifier), the order would be

Cyan,

then (anything on initiative count 20),

then Billy,

then Grok

I think it is the equivalent of rolling for initiative but it's automatically 20 and acts on that turn in the initiative order.

So, I think it works like

1. Demon Lord with Initiative 25

2. Human Fighter Initiative 21

3. Initiative count 20

4. Wizard with an initiative of 12

Edit: Hmmmmmm.... I suddenly realize it looks like I copied the above post...

Thanks!