What he meant by that, and this is truly insightful considering that it was mid 19th Century when he wrote that, is that if labor costs rise, those with capital will use their capital to buy machines that will replace labor; thus the cost of labor will be held in check. The affect on labor of course is to hold down wages! Today it is robotics!!! Marx was truly brilliant, but he didn't foresee that human nature would make it impossible for communism to succeed beyond the family or tribe level.
Malcolm Muggeridge , the British journalist, satirist, self-styled philosopher and Christian convert, said one time that all of human endeavor can be laid to vanity, greed, and instinct. We Homo sapiens don't want to acknowledge this unattractive feature of our species -- because of our vanity, I suppose. We like to think of ourselves as being on a higher plain than the other animals, and not merely as one of the many species that inhabit the Earth.
One of my favorite Muggeridge-isms is "Only dead fish swim with the stream."
https://en.wikipedia.org/wiki/Malcolm_Muggeridge