A gerund is a verb with -ing added to it. The word is then used as a noun. For example, start with the verb, ski. Then add -ing to the word and form, skiing. Then use this word as a noun.
Skiing is fun.
Running develops and strengthens the heart. Clear writing is essential in business. Seeing is believing. When the word is used as an adjective, it is a verbal. For example, start with the verb, run.