@Peachey and @Alan have cleared the doubt what a Gerund is!!

I just want to share an Article which may help you to know it better through examples---- 

https://www.italki.com/article/314/verb-patterns-in-english-gerunds-and-infinitives

Hope this helps:)

I'd just add that the difference between a "gerund" and "present participle" is not very important to a learner. You can call them both "-ing verbs", and this would never lead to a mistake (many modern grammar guides do exactly this).

For example, this statement is incorrect:

The main verb in "I am swimming" is a gerund

...but really: who cares? The sentence "I am swimming" is fine, regardless of whether you call that -ing verb a gerund or a present participle. Gerunds and present participles always look identical, so confusing the two is a totally inconsequential mistake.

I personally recommend you remove the words "gerund" and "present participle" from your vocabulary and replace it with the more practical "-ing verb".

When do I have to use gerunds?

A gerund is "a verb as a noun", and can take the place of a noun. This means it can be the subject or object of a sentence, and it can follow prepositions. We make gerunds by adding +ing to the verb.

Are gerunds similar to present progressive, regarding their function?

This is the confusing part. What you see in present progressive is [to be] + [present participle]. Present participles look exactly like gerunds: both add +ing to the verb. However, they function in completely different ways.

As I've already written, gerunds work as nouns. They can stand alone.

Present participles cannot work alone. They always need another part of the sentence to function. We make progressive forms by adding [to be]. We can use present participles as adjectives (=it needs a noun). You can add extra information by using a participle phrase (again, it needs a noun).