Unless the Lord builds the house, they labor in vain who build it; Unless the Lord guards the city, the watchman stays awake in vain.
It is vain for you to rise up early, to sit up late, to eat the bread of sorrows; for so He gives His beloved sleep.
(Psalm 127:1-2)
Vain is repeated 3 times in two short verses. It means useless. It can even carry the idea of being idolatrous.
Let me paraphrase:
It is a waste to pour yourself into a plan God isn't part of.
It is foolish to trust anyone else for your security.
God never meant for you to work yourself to the point of stress and death.
He will lead. He will defend. He will provide.