Typedef void (* Fun) (void)

Typedef void (* Fun) (void) Fun
Similar
Typedef int

After the declaration, fun can declare other variables as data types.
The exact meaning of fun is a function pointer type, which has no parameters and no return values.

Defines an int type variable I;
In this case
Typedef int;
Indicates that you have defined an integer data type int, which is equivalent to int.
So: int II;
Same as above, it indicates that an int type variable II is defined;

Similarly:
Void (* PFn) (void)
Defines a function pointer pointing to the function entry address similar to void Foo (void ).
In this way:
Typedef void (* Fun) (void)
Indicates that you have defined a function pointer data type.
While
Fun PF;
Define a function pointer PF. Change the function pointer to a function similar to void Foo (void ).