foo: function(){} code example

Example 1: what does $foo mean

It's just a placeholder

Example 2: what mean foo in programming

Foo (pronounced FOO) is a term used by programmers as a placeholder for a value 
that can change, depending on conditions or on information passed to the program. 
Foo and other words like it are formally known as metasyntactic variables. 
It can be helpful to use metasyntactic variables when creating sample code 
because programmers don't have to create unique names for each variable value.
Nonsense placeholders make it easier to focus on the code's core concept 
and functional goal.

When defining a template for creating a new user command, for example, 
the syntax for the command could be written as:

Command foo (arg1, arg2)