Every day we interact with objects, places and sensations that affect the way we think and feel. This can be used to the writer's advantage by planting symbols in the reader's path to reinforce a specific message, feeling or idea.
Look at the setting and the character's state of mind, and then think about what you want the reader to see. Is there a descriptive symbol or two that works naturally within the scene to help foreshadow an event or theme, or create insight into the character's emotional plight?
Mud slide/rock slide
Road sign barriers
Parking space lines
Strung rope or wire
Protected parks and designated areas
Border/Customs between countries
Great wall of China
*Barriers can also be economic, educational, physical health restraints, mental limitations (fears, phobias) and popularity (money, degrees, handicap, memberships, notoriety)
These are just a few examples of things one might associate with Borders/Barriers. Some are more powerful than others. A concrete barrier wall is a strong symbol, and likely will not require reinforcement. However, the presence of a baby gate in a home might not foreshadow a border/barrier on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.