Ring utilities are used to apply solid box-shadow to an element.

• Use the ring-{width} utilities to apply solid box-shadow of a specific thickness to an element.
• Rings are semi-transparent blue by default, similar to the default focus style used by many browsers. You may use ring-{color} utilities to control the color.

Check these docs for more details:

• Ring Width - Tailwind CSS
• Ring Color - Tailwind CSS