The letter that prefixes each ner_tag indicates the token position of the entity:

B- indicates the beginning of an entity.
I- indicates a token is contained inside the same entity (for example, the State token is a part of an entity like
  Empire State Building).
0 indicates the token doesn't correspond to any entity.

Preprocess

The next step is to load a DistilBERT tokenizer to preprocess the tokens field: