what mean rounded in bootstrap code example

Example 1: bootstrap 5 rounded

<img src="..." class="rounded" alt="...">
<img src="..." class="rounded-top" alt="...">
<img src="..." class="rounded-end" alt="...">
<img src="..." class="rounded-bottom" alt="...">
<img src="..." class="rounded-start" alt="...">
<img src="..." class="rounded-circle" alt="...">
<img src="..." class="rounded-pill" alt="...">

Example 2: bootstrap 5 rounded

<img src="..." class="rounded-0" alt="...">
<img src="..." class="rounded-1" alt="...">
<img src="..." class="rounded-2" alt="...">
<img src="..." class="rounded-3" alt="...">