If you know what a vector is, then you will be familiar with resolving a vector into the components in different directions.
For example, gravity near the surface of the Earth is a vertical vector. But, you can decompose that vector into, say, components tangential and normal to an inclined plane. And, in fact, you are free to decompose the vector into components in any pair of directions.
A quantum state is in fact an infinite dimensional vector. And superposition is the term used to describe its decomposition in a given basis of states.
Many people labour under the delusion that some states are in superposition and some are not. But, that is as absurd as saying some vectors can be decomposed and some cannot.
In short, given any state and any set of basis states, the state is in a superposition of those basis states. If you think, therefore, that superposition just means linear combination, then you'd be correct. Of course, if the basis you choose has the state in question as one of the basis states, then the superposition is trivial.
The role that linear algebra plays in QM goes much deeper than that. But, that's what a superposition is.