DAX table functions and filtering greatly enhance the complexity of calculations that can be done with DAX. The flip side is that formulas can become quite long. More importantly, with all the different contexts in play, it can be problematic to obtain correct results.

DAX variables make life much easier when designing advanced DAX code. The name is somewhat odd, as the purpose of DAX variables is that you can evaluate something once, and use it later in other circumstances (other contexts, typically) without worrying about the evaluation of the variable. In other words, a DAX variable is used as a constant!

A variable is declared using the VAR keyword. Multiple variables can be declared, and the declaration of a variable can use the value of another variable declared earlier. Declarations of variables are closed off by the RETURN keyword:

VAR ThisValue = 5
RETURN
...

It is good to know that DAX variables can be used in any expression in a DAX formula...