the meaning of code

for utterance in dialog["utterances"]:
history = utterance["history"][-(2*args.max_history+1):]
for j, candidate in enumerate(utterance["candidates"][-num_candidates:]):
lm_labels = bool(j == num_candidates-1)
instance = build_input_from_segments(persona, history, candidate, tokenizer, lm_labels)


Hello, I am new to transfer, how to understand these sentences?