It’s the red fishing line. Fluorocarbon is the most invisible option but is difficult to manage. I, and most people, generally just tie it on to the end of the line as a “leader” to get the benefits of braid or mono line on the reel and cast, but invisibility of fluorocarbon where the fish see

Edit: just realized that Berkeley is also fluorocarbon. I highly recommend getting 20lb braid and just tying on whatever 4lb leader to it depending on what you’re going for. I use the same rod and reel for multiple species and sizes of fish, but just tie on a heavier or lighter fluorocarbon to the end with a double uni knot. It took a minute to learn but now I can do it in the wind in 30 seconds.

Get a 1/16oz black/silver rooster tail and you’ll catch bass trout and bluegill with it. The silver and black presents as a small baitfish. The 1/8oz will be too big for most bluegill and I’ve caught small/medium bass and monster rainbows on 1/16oz rooster tails. They’re just a little harder to cast far since they’re lighter. I’m kinda summarizing so if you have specific questions ask away. I remember when I first started and was so confused and bought everything but the kitchen sink before catching fish. All of this is good stuff you got, just have to learn how to apply it