It means if the upper (上) beam (梁) is not (不) straight (正), the lower (下) beam (梁) will slant (歪).
And if you notice, the word for slant 歪 is made up of two words, not straight 不正, one on top of the other.
Such is the beauty of the Chinese language.
And it's poetic too.
I found it interesting that is has a 4+3 meter. I tried to find out the implication of its unevenness but couldn't find anything yet.
What I notice is that Chinese sayings usually have matching meters, i.e. 3+3, 4+4 or 5+5.
But in some sayings, the second portion has one less. And I feel it denotes the negativity and incompleteness of what it wants to convey.
What do you think?
Like in this case, if the upper beam is not straight, if the top is not set right, whatever that is the bottom will not be stable and may fall.
What other such uneven metrics of Chinese sayings come to mind?