Doubt about a point in BERT paper

In the BERT paper it says that during training it mask a fraction of the words and replaces them with random words:

The training data generator chooses 15% of the token positions at random forprediction. If the i-th token is chosen, we replace the i-th token with (1) the [MASK] token 80% of the time (2) a random token 10% of the time (3) the unchanged i-th token 10% of the time.

I can't wrap my head about the explaination …

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