The phrase "thick borders" refers to a situation where there are strict limitations or restrictions in place between different places or groups. It means that it is difficult for people or things to cross from one side to the other. Full definition
These templates have thick border lines around them, so they will be easy for your students to cut out their homework sticker charts themselves. (uniqueteachingresources.com)
The paint also forms thick borders around the canvases, creating a «frame» of painting, made out of the paint itself. (museumpublicity.com)
The front is dominated by the 7 - inch screen with thick borders on all sides. (gadgets.ndtv.com)