newest_folder = max(modified_times, key=modified_times.get) 

import os
# This script populates a dict with {sub-folder:time it was last modified} for all sub-folders in a given folder.
# It then finds the sub-folder with the most recent modified time
target_path = '/Volumes/Particular-path-of-interest/'  # Actual path has been redacted for posting
os.chdir(target_path)
folders = os.listdir('.')
modified_times = {}
for folder in folders:
    if os.path.isdir(folder): # MacOS puts hidden files in folders which can affect results if not filtered out
        modified_times[folder] = os.path.getmtime(folder)
# All the code above I'm fine with...
newest_folder = max(modified_times, key=modified_times.get) # This is the line I don't understand
print(newest_folder)

1. The function max takes an iterable
   
2. Iterating over a dict, yields the keys. Just try: list(modified_times)
   
3. For each iteration the key function is called with the key form the dict as argument. In this case the folder.
   
4. Items are sorted by the return value of the key function.
   
5. Every function in python is first class, because they can be passed around like any other object. First class objects in a language are handled uniformly throughout. They may be stored in data structures, passed as arguments, or used in control structures.
   

list_of_strings = [...]
max(list_of_string, key=len)

Quote:get(key[, default])

Return the value for key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never raises a KeyError.