Meaning of dot (".") in front of certain arguments in the Tidyverse reference

There are useful explanations here as well:
https://forum.posit.co/t/what-is-the-difference-between-and-data/76330

I have got some questions about it:

What does it mean ?
Sometimes I got an error:

object of type closure is not subsettable

Why does it pop up sometimes ?

Can you please elaborate a bit more clearly what is the difference between parameters and arguments ? Maybe with adding some examples.