What is the meaning of $ in a variable name?

There is no special meaning for a $ in a variable’s name.

While technically allowed, starting the variable name with a dollar sign goes against convention, generally used only by code-generators.

To quote the Java Tutorial by Oracle:

Variable names are case-sensitive. A variable's name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign "$", or the underscore character "_". The convention, however, is to always begin your variable names with a letter, not "$" or "_". Additionally, the dollar sign character, by convention, is never used at all. You may find some situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it. A similar convention exists for the underscore character; while it's technically legal to begin your variable's name with "_", this practice is discouraged. White space is not permitted.

$ is used internally by the compiler to decorate certain names. Wikipedia gives the following example:

public class foo {
    class bar {
        public int x;
    }

    public void zark () {
        Object f = new Object () {
            public String toString() {
                return "hello";
            }
        };
    }
}

Compiling this program will produce three .class files:

• foo.class, containing the main (outer) class foo
• foo$bar.class, containing the named inner class foo.bar
• foo$1.class, containing the anonymous inner class (local to method foo.zark)

All of these class names are valid (as $ symbols are permitted in the JVM specification).

In a similar vein, javac uses $ in some automatically-generated variable names: for example, this$0 et al are used for the implicit this references from the inner classes to their outer classes.

Finally, the JLS recommends the following:

The $ character should be used only in mechanically generated source code or, rarely, to access preexisting names on legacy systems.