instrument vs line in

[Dubickisbro]

so what exactly is the difference between plugging my guitar into line in or plugging my guitar into an instrument port?

 

[Steenamaroo]

A DI generally takes an instrument level in, and bumps it up to a line level output.

Instrument pickups are generally passive devices that only generate a very small voltage from magnetic differences created by vibrations in the conductor or the field surrounding it... These need to be amplified to a useable level...

The same holds true for a dynamic microphone, and to a similar degree to a condensor mic (which is not passive in design, but still need additional amplification)

It was also mentioned in that thread that instruments aren't balanced, where often line level inputs are.

Line inputs expect a higher voltage (hotter signal), so your guitar would be quiet.