Can someone explain to me what impedence is?
Also, I have a Presonus Firepod. It has three kinds of inputs: mic, line and instrument. What is the difference between them and does it make a difference for sound quality if I plug a mic into an instrument ijnput, etc?
Impedance is an electrical term that basically describes opposition to A/C current. It's kind of the same thing as resistance is for a DC circuit, but impedance is for an A/C circuit. You can read more about it here:
http://en.wikipedia.org/wiki/Electrical_impedance
What you basically need to know for audio equipment is that different types of connections have different impedance, as well as different operating levels. The equipment that you hook up to these connections expects to see a certain level and impedance, and if they are mismatched, you can get distortion, or very low signals, or just really bad tone quality.
Mic levels are very low, which is why they need to go through a microphone preamp to bring them up to "line level". Your microphone input on your Firepod feeds the microphone preamp built into that interface. If you try to hook up your microphone to a line level input, the signal level will be way to low, and you'll have to increase the gain somewhere, which will add unwanted noise and distortion.
Instrument inputs, also called DI (direct inject) are special connections that usually have a much higher impedance design to interface directly with electric guitars and bass instruments. The output of the DI usually goes through the preamp stage since these instruments usually need some pre-amplification to bring them up to line levels. If you hooked up your guitar or bass directly to the line level input, besides not getting enough level, the tone would also suffer due to the lower impedance.
Also, note that there are two different "line levels" in the audio world. There is the "professional" level, that is specified as +4 dBu. While the "consumer" level is specified as -10 dBV. Many "semi-pro" pieces of studio gear output signals at -10 dBV, such as most keyboard and rack mount synthesizers, drum machines, and the like.
So, that can be confusing when you have some equipment that is set up for -10 dBV and other equipment that is set up for +4 dBu, as you'll get some noise and level issues when trying to interconnect them. Many professional mixers have inputs that can accept either level, or some have separate -10 and +4 inputs.
To complicate matters even further, there are also "balanced" and "unbalanced" connections. But, we won't go into that now.
Just try to be aware of what type of connections your equipment has and to match the inputs and outputs of your various equipment the right way.