I wrote this for a non-marxist, so the language here is meant to be accessible for somebody who understands basic economics
To earn a living, workers sell their capacity for their labour (that makes commodities) to the capitalist. This creates wages.
To make a profit, the wages must be less than the actual value of labour (how much the commodities are sold for), but still high enough for the workers to decide to work for the company. This means that the price of a commodity is generally relative to wages, and equal to the value of labour that produced it.
What if commodities are produced more efficiently by machinery? This increases productivity and supply, but workers can’t buy back enough of these commodities (many of which are necessary, leading to strikes and poverty), so their wages are increased and prices are reduced further to counteract this, reducing profit.
Increasing wages of the working-class increases the demand for necessaries, this partially compensates for the loss of profit, but not fully. This means that the earnings of capitalists decreases, and thus the demand for luxuries decreases. This causes the investment into necessaries, increasing supply, and reducing the price of necessaries. This results in the profit of the economy decreasing overall
Of course, a single company switching to machinery doesn’t cause this. This is when the economy as a whole switches to machinery.
This links in to the prices being determined by the quantity of labour, though different types of labour (skilled vs unskilled) have different values