Question

explain space-time in simple language

Answer 1

Scientists believe that there exist 4 dimensions. The fourth is time. Yep, time is taken as a dimension.
Now, think of space time as a fabric that bends when an object with mass is placed on it.
Simply: Think of space time as dupatta that bends when an object with mass is placed on it.

EmoLord.

Answer 2

The term "space time continuum" refers to the interpretation, induced by special relativity(Einstein's first theory, for which he became a big deal), of time and space as coordinates in the same 4 dimensional space (what we call "Minkowski space" in physics). So instead of a 3d system (with x y and z axes) that evolves in time, you just have a 4-dimensional system, with x, y, z, and t axes. It doesn't evolve in anything; it's just... there. All at once.