The Mic/Line Switch allows you to either use the channel for a microphone or a line level (line level devices typically send more electrical current than a microphone. Examples of line level devices are keyboards, CD players, etc.).
Your mic input will be the female XLR connector (sometimes called a jack); the line input will be a female 1/4″ jack connector.
As a vocalist, your mic will enter the mixing board in one of these inputs.
Your microphone will almost always connect with the mixing board using XLR to the Mic input.
If you’re using a vocal processor, it may be a mic or line device.
That is, you will connect your mic to the processor and then you will connect the processor to the mixing board into either the mic or line input.
Note: if you don’t know if your processor is a Mic or Line device – try the Mic input first starting with the Gain as low as it can go.