Panel lining is when you paint the small lines in the plastic parts of Gunpla. This makes them really pop and makes your Gundam look larger and more realistic. This is a simple but very effective way to improve your builds!
Pannel lining is pretty simple. The hardest part is finding which tools suit you!
There are different colors of panel liners. Black should be used for grey and other darkly colored parts. Grey should be used for white and blue, and brown for warmly colored parts. If you don't want to get all the colors, just get grey; It's the most versatile.
This video will teach you more about how to panel line your gundam.