5x means 5 times a number. 5 is called a constant. x is called a variable. You can use any letter to represent a variable. Letter’s from a-z and also they are different from capital letters too A-Z. You can also use Greek letters to represent variables α - ω. But these letters are commonly used for angle variables.
When should I use 'depends'?
The only time you should use 'Depends' is when your package is an add-on or companion to another package, where your end user will be using functions from both your package and the 'Depends' package in their code.
What does the 'X' represent?
The 'X' represent here the multiple(times) concentration of any solution, FOR eg. 1X is some value of concentration while 2X will be double the concentration of 1X and the 5X will be 5 times concentrated than 1 X.
