I was watching this video where the user explains that he is using Raspberry Pi to trigger the relay board, which requires 5v.
But he says it's okay do it with the 3.2V that's from GPIO.
Does doing this harm the relay board?
The relay board in question has to power inputs, VCC and JD-VCC. The person tells it's okay to put 3.3V on VCC. But you still need 5V on JD-VCC.
This 3.3V is only used to power the opto-couplers.
If 3.3V were not to be enough, the opto-coupler would work, and the rest of the circuit won't do anything. But nothing bad will happen.
It is actually a good thing to power it using 3.3V as powering it using 5V might make the relay trigger even if the GPIO is set to HIGH. Please follow his advice.
Even more info can be found in this answer