I need to know what x is

X is the variable that is put into place for an unknown number

The letter "x" is often used in algebra to mean a value that is not yet known. It called a "variable" or sometimes an "unknown". In x+2=7, x is a variable, but we can work out it's value if we try! A variable doesn't have to be "x", it could be "y", "w" or any latter, name or symbol.