Spacetime is a mathematical model that combines space and time into a single interwoven continuum. Spacetime is not an object; rather, it is a structure that one can use to describe and measure relationships between events. It was first proposed by Hermann Minkowski in 1908.
In the simplest form of spacetime, known as special relativity, the structure is flat. This means that there is no preferred frame of reference; all frames of reference are equally valid. The flatness of spacetime leads to the famous result of special relativity known as the twin paradox: if one twin goes off on a journey and returns home, they will find that the other twin has aged more than them.
More generally, spacetime can be curved. This happens when there is matter or energy present; the curvature corresponds to the amount of matter or energy present. The most famous example of a curved spacetime is general relativity, in which gravity is described as being caused by the curvature of spacetime.