Aug 23 - Why do you use a next to nadar when you say "Voy a la piscina a nadar?"

I am just trying to construct a sentence, and one of my friends said this was the right way to say something like this. I don't get why. Could you guys maybe tell me why and give some examples too of something like this so I could get this topic a little bit more? Sorry if this question confuses anybody, I'm not an Español wiz or anything!