What is the blue line and the space appearing in Indesign document

Sharing a sample document would help to find the exact answer rather than us guessing what it might be

-Manan

It's hard to tell from the small snippet, but it looks like a line under the paragraph, assigned in the paragraph style under Paragraph Rules (below) or Paragraph Border.

That or as JJ said, just a guide line. Does it disappear when you switch to Preview Mode?

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In addition to the above question:

The blue line automatically appears. How can I remove it?

Thank you.