.Strat Brat,
Think of impedance in terms of water flowing through a pipe. The larger the pipe, the greater the volume of water that can pass through the pipe, from minimum flow to maximum flow, without backing up. You never want the incoming flow of water to exceed the flow capacity of the pipe.
The XLR jack is low impedance (2.4 kOhm). This is mic level - all mics with XLR cables are low impedance.
The 1/4" TRS phone jack is high impedance (>22 kOhm). This is line level.
The Input H "guitar" switch converts the 1/4" phone jack impedance from 22 kOhms to 1 mOhm (1 million Ohms). This allows for either plugging the guitar in directly, or running the guitar through a bunch of effects pedals first. The guitar itself is high impedance (typically 40kOhm), and each effects pedal adds its own high impedance to the signal flow.
It's okay generally to run a low impedance device (like a mic) into a high impedance input. The only downside is lower sound level/more noise in the mic signal than there would be when pairing the low impedance mic with the low impedance input.
Musical instruments are all high impedance. The proper way to record them on the DP-xx is as I described in my post above. The reason is that when a high impedance source is connected to the high impedance TRS phone jack and the Trim Pot is ~not~ in the "Line" position, the impedance is set to mic level and the mismatch can result in loss of high frequencies and other forms of signal distortion. Also, if the musical instrument has a pre-amp to boost its output signal, the impedance mismatch can place unwanted stress on the electrical components of the input that can result in component failure over time.