It's speculating that the rate of bitcoin appreciation in fiat terms will exceed the rate of fiat devaluation and cost of servicing the debt.

Unfortunately unlike Michael Saylor stackers can't issue more equity to buy bitcoin, betting that the price of bitcoin will outweigh the dilution. But they can take on debt to buy bitcoin like Saylor does. However, it is a risky proposition because corporations are structured to protect shareholders from personal responsibility in the event of a bankruptcy, and individuals are not and you can most certainly not borrow at rates as low as Saylor and with the types of terms Saylor gets.

As an individual to execute this you would need to be highly confident in your income, ability to borrow at reasonable rates and ability to service the debt comfortably. You don't want to have to sell your bitcoin at the bottom of a bear market to meet debt obligations.