Meaning of ~. (tilde dot) argument?

This is a formula, in a shorthand notation. Try this:

plot( mpg ~ cyl, data= mtcars )

The left hand is the dependent variable, the right hand is the independent variable. Much like y = bx + c means that y ~ x.

Formulas are one of the corner stones of R, and you will need to understand them to use R efficiently. Most frequently, formulas are used in modeling of all sorts, for example you can do basic linear regression with

lm( mpg ~ wt, data= mtcars )

...to see how mileage per gallon depend on weight. Take a look at ?formula for some more explanations.

The dot means "any columns from data that are otherwise not used". Google for "R formulas" to get more information.