They were sitting by a swimming pool. Here the underlined word is a -

Verb এর সাথে ing যোগ হয়ে যদি noun এর কাজ করে অর্থাৎ Verb ও noun এর কাজ করে, তাহলে তাকে Gerund বলে।
সংক্ষেপে: Gerund = Verb + ing = noun = Verb + noun.
এই বাক্যে 'swimming' শব্দটি Gerund হবে।

- Sometimes it is difficult to distinguish between a present participle and a gerund, since gerunds can also be used as adjectives.
For example:
In the phrase 'swimming pool' the word 'swimming' is a gerund used as an adjective.
- We know this because, swimming pool means pool that is used for the activity of swimming.
- It is not a present participle because the pool is not doing the action of swimming.

Lets see another example:
- He dived into the swimming pool.
- He waved to the swimming children.

In the first sentence, the pool is used for swimming. So it is a Gerund.
In the second sentence, children are doing the action of swimming. So it is a Present Participle.

------------------------
Here are some other examples of gerunds used as adjectives:
- Sewing machine
- Frying pan
- Baking sheet
- Fishing rod
- Bathing suit
- Drinking water
- Shaving cream
- Ironing board

Source: Gmat Club and Applied English Grammar and Composition, P.C Das