What is red stroke on board?

Hey! It looks like you have rulers visible (see this page: https://help.figma.com/hc/en-us/articles/360040449713-Add-guides-to-the-canvas-or-frames#View_distances_redlines).

You should be able to hide them with the shortcut Control + G. If you have rulers showing, you can also click on the red line and delete it with the backspace key.