What is mental barrier

Your mind cannot conceive of a resolution, or a path forward, to deal with a situation, person, place, or thing.

An example was running the 4 minute mile. For a long time it was "thought" impossible, so no body did it, After one man proved it was possible, then it started to be a norm for runners.

It is a place or thing your mind tells you is not possible.

"I want to try something new but am afraid I'll fail. Therefore I won't do it."
"A friend of mine was hiking and saw a snake. I don't like snakes so I'll never go hiking."