what does the join function do in python code example

Example 1: Concatenate Item in list to strings

>>> sentence = ['this','is','a','sentence']
>>> '-'.join(sentence)
'this-is-a-sentence'

Example 2: join() python

# example of join() function in python

numList = ['1', '2', '3', '4']
separator = ', '
print(separator.join(numList))