ner transformers implementation in keras

Question

I am following this keras link https://keras.io/examples/nlp/ner_transformers/

to train my own NER model. I am not clear why we have tags +=1 in the following function

def map_record_to_training_data(record):
    record = tf.strings.split(record, sep="\t")
    length = tf.strings.to_number(record[0], out_type=tf.int32)
    tokens = record[1 : length + 1]
    tags = record[length + 1 :]
    tags = tf.strings.to_number(tags, out_type=tf.int64)
    tags += 1
    return tokens, tags

Could anyone help clear this?