Hi @jianying-zhang. Thank you for your question. This is an example of marking up a corpus, and it stands for sentence boundaries, with <s> indicating the beginning of a sentence and </s> the end of the sentence. Another example you might encounter is <p> and </p> for paragraph boundaries. You can ignore them, but if you find them distracting you can turn them off by following the instructions below:
- Click on the View options button
- Untick the s in the Show structures box
Hope this helps.