Answers for "what is the meaning'DIRS': [os.path.join(BASE_DIR,'template')],"

'DIRS': [os.path.join(BASE_DIR, 'templates')],

Incorrect syntax in polls app tutorial by Django

Use this: 
os.path.join(BASE_DIR, 'templates')

instead of this:
BASE_DIR / 'templates'

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': [os.path.join(BASE_DIR, 'templates')],
        'APP_DIRS': True,
        'OPTIONS': {
            'context_processors': [
                # codes...
            ],
        },
    },
]

'DIRS': [os.path.join(BASE_DIR,'template')],

BASE_DIR /"challenges"/"templates" 

'challenges' is the app name, it will automatically look for the app name and the
 folder inside naming 'templates'

And if all the template and static files are in app folder, then you only need to 
register the app in setting.py, Under the hood, django will autoamtically lookup for
the file named template and static in that app folder