<div class="border-solid border-2 border-sky-500 ...">
  creates a solid border, with width of 2, skyblue color
</div>

Solid border

"border" // defaults to solid
"border-solid"
"border-dashed"
"border-dotted"
"border-none"

Border radius (rounded corner)

"rounded-sm" // 2px
"rounded" // 4px
"rounded-md" // 6px
"rounded-lg"
"rounded-xl"
"rounded-3xl" // 24px
"founded-full" // 9999px

Border width (thickness)

"border-0"
"border" // 1px
"border-2" // increments by 2
"border-8"