It is from the Bible. Look there and read the rest of the scripture.
Plainly it means that the way to eternal life is a narrow path. The same as a narrow path up a mountain. All steps outside that path leads to destruction. Eternal death. The same as walking outside of the safe path up the mountain.
It is talking about there being a large amount of ways to death and the way to life is a narrow way. Much like one wire in a phone cable, there is only one that will work and all others will not.
When the last Unicorn Died.
No, I know nothing of poetry. I just write what comes.