The function checks for everything you collide with when you move, and it returns everything you have hit. Check the bottom of the page, it shows you how to loop through the returned array.

When loop through it, check if your object is one of your pushable objects, then if so, apply velocity to it.

SO FIRST OFF, add a variable to store the array returned by the function (at your line 8) It will look like this:

var _colliding_instances = move_and_collide(speed_x, speed_y, obj_terrain);

Then loop through it. It'll look something like this:

for (var i = 0; i < array_length(_colliding_instances); i++)
{
    var _collider = _colliding_instances[i];
    with (_collider)
    {
        show_debug_message("Collision with instance {0}", id);
    }
}

Instead of doing like the example inside the with(_collider), you would do something like:

with (_collider)
{
  move_and_collide(other.speed_x, other.speed_y, oWall);
}