Variable in programming language are often regarded as 'storage box' in the sense that, they held values of certain items (user inputs, defined values , etc.).
Php as a language has her own way of storing variables, the symbol $ if used to identify or define a variable.
Take this example
<?php $greet = "hello world"; ?>
this little piece of code shows a php opening tag <? php (to know whats tags learn Html) and later the variable $greet which was defined using $ symbol, miss that and your code will be really messy, followed by the assignment operator = through which we save hello world into the variable $greet in quote. The quote shows that the value we save is a string, and finally the php closing tag.
Next lesson will work more on how variables work