system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is Spacetime and Curvature of Spacetime ?

I googled, but they were too hard to understand. So please explain like I'm five ...
gpt
Space has 3 dimensions. Objects can move up/down, left/right, forward/back through space and we can describe their motion. Similarly objects can move in time, and we can think of time as a dimension as well that objects can move through. Spacetime is simply the combination of these two concepts. It is a 4 dimensional 'space' with 3 regular spatial dimensions, and 1 time dimension.

Imagine now that you're a bug on a flat metal disk. The temperature of the disk changes but the disk itself is flat. You also have a metal ruler with you so that you can make sure that you're travelling in a straight line. As you walk across the hot disk your ruler will heat up and expand which means the lengths on it will change. If you're relying on your ruler to help you walk in a straight line you'll end up walking in a curve because your ruler has heated up slightly and you're now no longer able to measure distances with respect to a standard unchanging ruler. This is analogous to spacetime curvature.
When you move through a gravitational field your 'ruler is heating up' so to speak. Your path through the gravitational field will look straight to you but it will 'actually' be curved according to observers.