you're talking about 2 different things.
some mics, like condensers, need a phantom power source (usually 48v) to work. many mixers "throw" phantom power, allowing condensers to work with them w/o needing an external power supply.
the other thing: internally, mixers work at line level, and most (all?) outputs from them are at line level. things to watch out for here, when connecting external gear, is whether the connection is balanced or unbalanced, and whether you're working at -10 or +4 (aka consumer and pro levels, respectively).