Flexbox

order: 0 (default)

CSS
.container {
  display: flex;
}

.box-1 {
  order: 0;
}
Box-1
Box-2
Box-3
Box-4
Box-5

order: <value> (default)

CSS
.container {
  display: flex;
}

.box-1 {
  order: 1;
}
Box-1
Box-2
Box-3
Box-4
Box-5

box 1 is last because first four box (2, 3, 4, 5) is order 0 and last box (1) is order 1, so like 0, 1

Arrange the Box

CSS
.container {
  display: flex;
}

.box-1 {
  order: 4;
}
.box-2 {
  order: 1;
}
.box-3 {
  order: 2;
}
.box-4 {
  order: 5;
}
.box-5 {
  order: 3;
}
Box-1
Box-2
Box-3
Box-4
Box-5

Suppose element have same order then arranged based on html element arranged order

CSS
.container {
  display: flex;
}

.box-1 {
  order: 4;
}
.box-2 {
  order: 1;
}
.box-3 {
  order: 2;
}
.box-4 {
  order: 1;
}
.box-5 {
  order: 3;
}
Box-1
Box-2
Box-3
Box-4
Box-5