Examples

Example 1

When Joe told his parents that he broke the window, they were surprised by his honesty, but they still grounded him for three months anyway.

Honesty is an abstract noun because it doesn't take up any physical space in the universe. You can't point to it, pick it up, or carry it to an alter as a ritual sacrifice.

Example 2

Our Flag Day party was a smashing success.

You can't see, feel, taste, touch, or smell success (unless it's in the form of an Oscar statuette), so it's an abstract noun.

Example 3

It takes a lot of courage to wear a fanny pack.

Courage is an abstract noun because you can't apply any of the five senses to it.