Derivative f '(x) is the extreme value of F (x) at x = x0. What does x in x = x0 mean? Does it mean a specific number or a variable?

Derivative f '(x) is the extreme value of F (x) at x = x0. What does x in x = x0 mean? Does it mean a specific number or a variable?

The derivative f '(x) is the tangent slope of F (x) at x = x0
X is the independent variable
X = x0 means that when the independent variable takes x0, x0 is a specific number
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