This argument indicates to the model which tokens should be attended to, and which should not.
For example, consider these two sequences:
thon

from transformers import BertTokenizer
tokenizer = BertTokenizer.from_pretrained("google-bert/bert-base-cased")
sequence_a = "This is a short sequence."
sequence_b = "This is a rather long sequence. It is at least longer than the sequence A."
encoded_sequence_a = tokenizer(sequence_a)["input_ids"]
encoded_sequence_b = tokenizer(sequence_b)["input_ids"]