What is spacetime?  

Spacetime is a way of looking at the fabric of the universe as an interconnected concept of space -- which we typically think of in in terms of three dimensional (3D) Euclidean geometries -- and time -- which we think of as an inelastic dimension that flows continuously at a fixed pace.

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These two aspects of our model of the universe are combined together in Minkowski Space.  This Minkowski Spacehttps://en.wikipedia.org/wiki/Minkowski_space is a 4 dimensional space whose definition ensures the spacetime interval between any two events is independent of the frame of reference.  This means that two observers will see the same event from different vantage points in agreement with the same physical laws and in agreement with Einstein's theory of Special Relativity.  The concept of spacetime allows the laws of physics to be invariant in all inertial reference frames and works in accordance with a single value of the speed of light in a vacuum (2.99792458E8 m/s)and thus spacetime is a central component of Einstein's theories.

Spacetime is mathematically a model of the universe in four dimensions.  The concept of a spacetime defines any event as a unique position at a unique time and defines a spacetime as the union of all events.  Mathematically we can represent a location in spacetime with four real numbers.

Spacetime is independent of the observer.  In other words the laws of physics will hold true and would result in the same representation of an event no matter what reference frame the observer is in when they observe spacetime.