You use x as the dummy parameter for g, but you do not use x within the formula.
In order for your code not to crash you would have to have defined x as symbolic or given x a value before you defined f, and it would be that symbol or that value that would be "captured" in the definition of f.
For example,
x = 5;
f = @(y) 3 + x + y;
x = 19;
f(7)
then the value "x = 5" gets "captured" when f is defined so even though you changed x after you define f, the value 5 will be used not the current x.
