x is a variable, a number that you don't know what it is.
Your mom puts M&Ms in your lunchbox. You haven't looked in your lunchbox yet today. There is a number of M&M's in your lunchbox (if mom forgot, then the number is 0), but you don't know what that number is yet. x is the number of M&M's in your lunchbox. It's a number, but you don't what the number is yet. If you open your lunchbox and find 3 M&M's (mom is weird), then you know that x = 3.