It's a variable declaration with an initialiser.

$ cat a.cpp
#include <stdio.h>

struct Point {
   Point(int a_x, int a_y) : x(a_x), y(a_y) {}
   int x;
   int y;
};

int main() {
   Point point(5,6);
   printf("%d,%d\n", point.x, point.y);
   return 0;
}

$ g++ -o a a.cpp && a
5,6
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It's like new, but the object is allocated on the stack instead of the heap, and the variable is a reference instead of a pointer. It's the object equivalent of

int i = 5;
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which can also be written as

int i(5);
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which is what allowed me to write

Point(int a_x, int a_y) : x(a_x), y(a_y) {}
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instead of

Point(int a_x, int a_y) { x = a_x; y = a_y; }
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It's a variable declaration with an initialiser.

Ah! Constructor initialization lists. Thanks.

I would like to thanks Ikegami for his answer. I am just a beginner myself on C/C++. I am sorry I couldn't answer your question like he does.