Understanding of y=f(x): y represents the function value and x represents the independent variable, that is, the independent variable x is calculated according to a certain correspondence.

A function value y, where x can be a letter or an algebraic expression, depending on the specific function.

For example, the x in f(x) = 6x2+2x+ 1 is the x in the function expression.

And the independent variable in f (x) = (x+y) 2-2 (x+y) is x+y, which is the x in the function.

To sum up, x in f(x) in y=f(x) is an abstract concept, which is embodied as the independent variable of the function.