(countable) A barrier is something that stops passage; it does not allow anything to get through it.
The barrier across the road stopped traffic from getting past it.
(countable) A barrier is an obstacle; it is something to prevents progress.
The bug in the code was a barrier to us completing the project on time.
(countable) A barrier is a boundary or limit.
The wall surrounding the jail was a barrier to all those who want to get out.

Noun