Spritely Happy Fun Time!

So I bought Terraria a while back on Steam when it was on sale for 4 bucks or something like that, but I didn't play it much because shortly after setting up a multiplayer server so my husband and I could play together, we exceeded our bandwith for the month and had no internet for about three weeks.

Terraria being on Steam, which requires the internet, I didn't play much again for fear of running up our bandwith. Until today, I decided to try it out. I started up a new world, and found with glee that all of the items my husband (who is a Terraria veteran) had thrown at me during our brief multiplayer stint stuck with me. So I had great weapons and armor.... but I had no idea how to play. So I'm working my way through a world save named Cherry Chapstick and I don't know what this thing is that I found under my house. And it's not very far under my house, either. Yes, I know I could look it up. But I am lazy in a strange way, and instead of taking 30 seconds to look it up on Google, I'm going to take five minutes + however long it takes you, my mythical readers, to respond, and ask if you know what this thing under my house is and how I get rid of it.

Yes, I know it's dark and hard to see. Please tell me what this thing is though.

Also, if you know of any reliable, easy to navigate Terraria guides, I'd be greatly grateful. (......?)

Greatly grateful????