Это словарное понимание эквивалентно:
# Set up a new dictionary to hold the result
d_new = {}
# Iterate over key/value pairs
for k, v in d.items():
# If the value is greater than 2
if v > 2:
# Append to the new dictionary as required.
d_new[k + 'c'] = v*2
Вывод:
>>> d_new
{'cc': 6, 'dc': 8}