Higher-level information that you eventually have to know. Optional reading if you’re very new and confused, but go and read about it sometime.
General advanced information:
Vongola succession
Varia Arc spoilers:
Vongola Rings
The Cradle Affair
Future Arc spoilers:
Ring Boxes
The Arcobaleno
Tri-ni-sette (7^3)
Motorcycles
Hell Rings
Choice
The Arcobaleno Trials
Vongola 101 