Phantom power (48V) is a power supply that many audio interfaces and mixers send over the microphone cable itself. Some microphones absolutely need it, others don’t — and for a third group it can cause damage in the worst case.
Which microphones need 48V
Condenser microphones absolutely need 48V just to produce a signal at all — without phantom power, the channel stays completely silent. This applies to most studio vocal and overhead microphones.
Which microphones don’t need it
Dynamic microphones (e.g. the common Shure SM57/SM58) don’t need phantom power and generally work fine even if it’s accidentally switched on.
Why ribbon microphones need caution
Older or unbalanced ribbon microphones can be damaged by phantom power. When in doubt, check the microphone’s manual before connecting to see whether 48V is explicitly allowed.
Where to turn it on
Phantom power can usually be switched on per channel on the interface (a button or software toggle, often labeled “48V” or “+48”) — only enable it for channels that actually have a condenser microphone connected.