I'm relatively new to the tenor, and I'm not sure what the "set up" is that I keep seeing on here. Can you help?

The set up is the brand of sax, the particular model of that brand, the type of pads, the type of resonators, the type of mouthpiece, ligature, and reed, the spring tension (light or heavy) and even the height of the cups. Just for illustration a legit player will likely have a different set up then a jazz player or a rock musician. I wouldn't worry about it too much. If you have a sax that plays well spend your time practicing and listening to good players. If you are a newbie you already have your plate full learning about music and learning about playing the sax without worrying about set ups. Just to keep things in perspective a friend of mine bought a soprano that belonged to a very famous jazz musician. My friend said the horn was a dog so he resold it and kept the case because it had stickers from all around the world on it. There is also a flute player in a famous orchestra who has the first chair in the flute section and he played on a student model flute which pissed off the other players with their zillion dollar solid gold diamond studded flutes. Have fun, play music.