Someone was saying that even if using machines becomes cheaper than using humans, capitalist will still use humans because
"automation constitutes constant capital and human labour is variable capital
The Tendency for the Rate of Profit to Fall disproves that fact"
What do those mean?
From wikipedia:
Automated machines thus are the constant capital. And human workforce constitutes variable capital.
Essentially, machines are not useful without human intervention. Machines can boost productivity of human labour, but they cannot produce value on their own regardless of their level of technical sophistication.