For a notice to theory of relativity the term space time often put them in turmoil. I was also wandered by this term when I started to read about theory of Relativity. This is a small step to make you understand about space time.
In physics, space time is a model that combines 3-D space into a single construct called the space-time continuum (in which the time plays the role of the 4th dimension). According to Euclidean space perception, our universe has three dimensions of space, and one dimension of time. By combining the two concepts into a single manifold, physicists are able to significantly simplify the form of most physical laws, as well as to describe the working of the universe at both super galactic and subatomic levels in a more uniform way.
i.e. 3-d space+1-d time=space time
Treating space and time on the same footing and as two aspects of a unified whole was devised by Hermann Minkowski shortly after the theory of special relativity was developed by Albert Einstein. This unification is further exemplified by the common practice shown by specialists of expressing time on the same units as space by multiplying time measurements by the speed of light. The concept of space-time is vital to this theory and also to general relativity, an extension of special relativity that takes into account gravitation.
Space-times are the arenas in which all physical events take place. Viz : the motion of planets around the sun may be describes in a particular type of space-time, or the motion of light around a rotating star may be described in another type of space-time.
The problem of the actual number of dimensions of our universe is still open, as some theories predict as many as 26. In these theories however all the additional dimensions are such that the universe measured along them is subatomic in size. As a result, even if the universe had many more dimensions, we would only perceive four of them.