Wikipedia defines superposition as:
"... a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state; and conversely, every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrodinger equation; since the Schrodinger equation is linear, any linear combination of solutions will also be a solution."
What that really means is that a quantum system can be in multiple states at the same time when not being observed. They can exist across all possible states simultaneously. Think of a windy day on a pond with lots of waves. No doubt you've seen them overlapping at some point. That is a brutally simplistic explanation of superposition in a nutshell.