What does "int* p=+s;" do?

Built-in operator+ could take pointer type as its operand, so passing the array s to it causes array-to-pointer conversion and then the pointer int* is returned. That means you might use +s individually to get the pointer. (For this case it's superfluous; without operator+ it'll also decay to pointer and then assigned to p.)

(emphasis mine)

The built-in unary plus operator returns the value of its operand. The only situation where it is not a no-op is when the operand has integral type or unscoped enumeration type, which is changed by integral promotion, e.g, it converts char to int or if the operand is subject to lvalue-to-rvalue, array-to-pointer, or function-to-pointer conversion.

Is array s decayed to pointer type?

Yes.

What does int* p=+s; do?

Unary + operator forces the array to decay to a pointer.

C++ Standard, 5.3.1 Unary operators(P7):

The operand of the unary + operator shall have arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument. Integral promotion is performed on integral or enumeration operands. The type of the result is the type of the promoted operand.

The unary + form (+s) forces the operand to be evaluated as a number or a pointer.

For more information, please see this stack overflow answer.

Test this:

#include <stdio.h>
int main(){
    char s[] = { 'h', 'e', 'l', 'l', 'o' , ' ', 'w', 'o', 'r', 'l', 'd', '!'} ;
    printf("sizeof(s) : %zu,  sizeof(+s) : %zu\n", sizeof(s), sizeof(+s) ) ;
}

On my PC (Ubuntu x86-64) it prints:

sizeof(s): 12,  sizeof(+s) : 8

where

12 = number of elements s times size of char, or size of whole array
 8 = size of pointer

That's a unary plus symbol which has no practical effect here. For example:

#include <iostream>

int main() {
    int a[] = {1};

    std::cout << a << " " << +a << std::endl;
}

This prints the same address for both a and +a. The array is decayed to pointer as usual.

Note that, if it had been an unary minus -a instead then GCC would show the error:

error: wrong type argument to unary minus

Edit: Though it has no effect in OP's code, a and +a are not exactly same. Please refer to the answers by Khurshid Normuradov and songyuanyao for details.