What does second parameter in ClickHouse function toDateTime64 mean?

ClickHouse has function toDateTime64() to convert string into DateTime64 data type.

Example from official documentation:

SELECT * FROM dt WHERE timestamp = toDateTime64('2019-01-01 00:00:00', 3, 'Europe/Moscow')

It takes 3 parameters:

1. Date string
2. Integer
3. Timezone

But there is no info about the second parameter. What does it mean?