"Nobody needs to swim."
Translation:Ingen behöver simma.
A modal verb is used to "indicate modality – that is, likelihood, ability, permission, and obligation" (Wikipedia: Modal verb). That is, it modifies the perspective on the main verb.
Among the very common modal verbs, we find behöver, borde, brukar, får, kan, måste, ska, vill and tänker, for example. I was unable to find a complete list though.