Niels is a Danish given name for boys. It belongs to a large family of names in many languages which are derivatives of Nicolaos (Greek).
It is one of the most common Danish names and has been for generations. Its patronymic, Nielsen, is the 2nd most common Danish surname.
The Norwegian/Swedish version is Nils and is pronounced exactly the same. (In Danish names it is a fairly common practice to add an e to another vowel for purely decorative purposes.)
It is one of those names which frequently form part of a double name, with Niels Peter, Niels Christian and Niels Erik being the most common combinations.