meaning of &variable (passed to function)

func being some arbitrary user defined function

It couldn’t be “arbitrary” – it must take a pointer to int or a void* in order for the call to be legal.

This ampersand is the “take address” operator. It passes func the address of a, so that the func could, for example, modify it:

void func(int *pa) {
    *pa = 4; // Note the asterisk - it "undoes" the effect of the ampersand
}

If your main prints a after the call to func, it prints 4 instead of 3.

Note that if you pass a instead of a pointer to a to a function that takes an int, not an int*, then modifications done to that int inside the function will have no effect on the parameter that you pass, because in C parameters are passed by value.

the variable a in the actual code is probably global or extern or something

It is probably not global, because there is no point in passing globals around: by virtue of being global, they are already accessible from everywhere.