Bootstrap allows columns within a Bootstrap row to be ordered responsively using the class: order-{bp}-{0|1|…|12|first|last}. For example, using order-md-2 gives the column order 2 when the screen size is greater than the md breakpoint value. Therefore the order- classes modify the CSS order property of the flex item.
e.g. Using order-md-2 gives the column order 2 when the screen size is greater than the md breakpoint value.
Class order-{bp}-first gives the column order -1 and class order-{bp}-last – gives the column order 13
The default column order is 0.
You can read more about Bootstrap Ordering here.