A variable is a container you can store a value in. When you use that variable in an algorithm, it opens up the container and looks at the value inside. This lets you write smarter algorithms that behave differently depending on the value stored in the variable.
For example, if you wanted to write an algorithm to say Happy birthday, you're turning 10! to my little brother, that's great now but when I run it next year I'd want it to say, Happy birthday, you're turning 11! I could create a variable called age to store my brother's age and write my algorithm to say Happy birthday, you're turning 'age' instead. Because variable can change, every year I can update the variable 'age' to equal 'age' plus 1.
In this puzzle, we're going to be using a variable that will set the length of the line our artist draws. Later on in our code, the move forward block will look at that length variable to see what value you've set it to.