It's not a stupid question. Without this forum I would have never known about them either! Basically, some plants release what are called tannins when put in the water. Specifically, Indian almond leaves, oak leaves, and some teas (red/roobios, possibly green). This mimics what naturally happens in slow moving rivers, lakes, and ponds when dry leaves fall in the water and decay. This lowers the PH and possibly has other benefits. While we like having clear water so it is easy to see our fish, they don't need clear water (not to be confused with CLEAN!).
So if you use carbon in your filter, you could never get tannins because the carbon will remove them, just like it does with certain medications.
I personally love the look of 'tea water'. Not super dark, but just a tint seems to enhance my fish somehow.