Spacetime, as a concept, is related to a space that consists of 4 dimensions instead of the regular 3-dimensional space. As early as 1905, Einstein proposed a now widely popular theory that the speed of light is independent of the motion of all observers, and that space and time are interconnected in a single continuum. This theory, which is now a cornerstone of modern and quantum physics, is known as Einstein’s special theory of relativity. Einstein’s proposed idea of a single continuum where space and time are interwoven is what people call “space-time”.
According to this theory, time—which has traditionally been considered an independent entity according to the principles of classical physics—is affected when a body moves through space. This happens because, according to the theory, time and space are connected and part of a single continuum—spacetime.
In this video, we discuss spacetime in absolutely simple words: what exactly is spacetime and how is it related to the force of gravitation and Einstein’s theory of relativity?

References:
https://asd.gsfc.nasa.gov/blueshift/i…
https://sci.esa.int/web/lisa-pathfind…
https://sites.pitt.edu/~jdnorton/teac…