What is initiative count?

[RumoCrytuf]

Stop me if this has already been asked, but what exactly is initiative count 20? I see it on a couple of creatures (like the beholder for example) and I have no idea what it does or means. Can someone explain?

[Sicarius Victis]

If something has "initiative count 20", it means whatever it does happens as if it had its own turn, and it had rolled a total of 20 for initiative.

[Rysto]

Treat it as though you rolled initiative for that effect and the result was 20.

In other words, it happens after the turn of a creature that rolled a 21 for initiative and before one that rolled a 19.