Blue Line NEXT TO row numbers

there is a blue line showing next to three row numbers in a worksheet i sent my temp via teams. It was not showing this before on my one drive copy. these three rows will allow me to enter data but any data entered into the second or third row will disappear as soon as you tab into the next cell. what is this blue line and how do i get rid of it?

Please attach a sample workbook (not a picture or pasted copy). Make sure there is just enough data to demonstrate your need. Include a BEFORE sheet and an AFTER sheet in the workbook if needed to show the process you're trying to complete or automate. Make sure your desired results are shown, mock them up manually if necessary.

Remember to desensitize the data.

Click on GO ADVANCED and then scroll down to Manage Attachments to open the upload window.