Take for example ceiling fans: the newer DC motor ones use up to 70% less electricity than conventional AC motor fans for the same size/output (although cost more to purchase).
In: Physics
To drive a motor forwards, you need to get electricity and push it through a thing, causing it to move.
AC voltage does from 230V (positive) to 230V (negative), meaning for part of the cycle, it’s literally at 0V. Only for a brief moment, since it does that whole cycle 50x a second. (Using the standards here, but it’ll be similar everywhere). Now when the voltage goes negative, it actually wants to pull the object backwards, so you do something in the motor or circuit to push it, and it kinda works, but it doesn’t do it very efficiently.
Switch to DC voltage, pinned at a set voltage, say 24V, it sits at +24V the whole time, pushing the same current through, in the same direction, consistently delivering power in a good way. Perfect.
The problem is that the electricity is delivered to your house in 230V AC, so you need a fancy bit of circuitry, a step down transformer, an AC to DC rectifier, and all that isn’t perfectly efficient. But it’s way better for the fan.
So you pay more, because there’s more components and stuff, but it’s more efficient than trying to power a fan with AC.
I hope that kind of explained it like you’re 5? Let me know if any of that needs breaking down more.
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