This video explains in detail and simple language what DI boxes are, their functions and why you need to have them as part of your audio setup.
Check out the other videos mentioned.
Audio Signal Levels: Mic, Instrument, Line, Speaker
https://youtu.be/6AqIzbFOxHI
BALANCED vs UNBALANCED Audio Connections Compared | How to Eliminate Noise from your System
https://youtu.be/IZsnURkBRoQ