what is align items unset in css code example

Example 1: how to align items in css

div
{
  display:flex;
  align-items:center;
  justify-content:center;
  
}

Example 2: flex align top css

align-items: flex-start | flex-end | center | baseline | stretch

.container {
  display: flex;
  align-items: flex-start;
}