Unless the Lord builds the house,
those who build it labor in vain.
Unless the Lord watches over the city,
the watchman stays awake in vain.
There are some houses that are built that do not have the Lord as the chief architect. He is not the one building it... but it gets built.
There are watchmen who stay awake all night and the city remains safe.
To understand this verse, first understand the word vain. It means empty. Why? Why is the labor in vain? Why is the watching in vain? because it is pointless. The only one who can build eternal things is God himself. When He builds a house, it lasts forever, when he watches a city, it is an eternal city, take up your citizenship there.