What is the usage of gerund? What word does it usually modify?

As the name implies, gerund is a word with the verb +ing as the noun, such as

Go+ing = go

sing

Since it is a noun, it certainly doesn't modify any words.

Their usage is generally the same as that of common nouns. See example:

Going home is always something to look forward to.

Singing in the rain can be a very exciting experience.

Smoking is harmful to your health.

I hope it helps you. If you are satisfied, please click "Choose Satisfied Answer" to adopt it in time. Thank you.