How do I get rid of a yellow highlite in iCloud Numbers?

I am in iCloud numbers. Sometimes a yellow highlight will appear over the regular black border in a cell. When I move my cursor over it my name appears in a yellow box. I have tried everything to get rid of it. If I change border colors the yellow is still there. If I delete the row the yellow outline just moves down to the next row. I do not even know how it got there. Anyone know what this is and how to get rid of it? Much appreciated! Thank you!

[Re-Titled by Moderator]