"Native Americans" are the groups of people who lived in North America before Europeans came. People who are descended from these groups and maintain some on their traditions are still called "Native Americans".
In years past, English speakers called them "Indians", but this was incorrect. Some people still call Native Americans "Indians", but in formal situations we say "Native Americans".
The "culture" of a group includes things like their:
- style of dress
- traditional food
- literature and stories