Everything we understand to exist exists within time and space. Time has no meaning or context without human perspective, but all things still progress forward in time without anyone watching or any implied meaning to people. Are we bound by time? Is God bound my time? Is time a creation?
The word "spacetime" comes from the concept of the 4-dimensional universe. You have width, depth and height, and then all matter exists within time as well. So spacetime just means space and time. What we've discovered and verified through experiment is that time doesn't behave exactly the same for everyone -- that is, one second for me is not one second for you, even if we use the same watch. The faster you move through space, the slower time appears to go. However, the effect isn't noticeable on our scale because we're all moving through space at about the same relative speed -- we're all on the same planet, traveling in the same direction, and measuring ourselves against the same sun, moon and solar system. So, to humans, everything seems to stay about the same. But if we put one Casio on Pluto and one on Mercury, after a few hundred years or so, we'd find the watches are off by several days. At higher speeds, the effect becomes very pronounced. We can also measure this in small, volatile, fast-moving particles by adjusting their speeds and seeing how long they last for. The effect is VERY consistent.
Time actually affects gravity because information can only travel through space at the speed of light. So the space-bending effects of gravity travel through space between planets, stars, and galaxies at the rate that light travels. That means that the pull of galaxies toward each other not only weakens with distance, it experiences a delay with that distance as well. Our own galaxy is being pulled toward where the Andromeda galaxy was many years ago instead of where it is right now.
Time is tricky because it behaves in ways that defy our sense of consistency in places that will never apply to us. You can't visualize it as, say, the water that the universe sits in or the strings that tie matter together. It doesn't act quite like anything else I know, and you're right to say that it's more of an intellectual construct that we use to describe the universe than a concrete thing.
Why are we talking about spacetime? Because Cathy Woods is putting together a space-oriented ETF. We are big fans of ARK and will certainly be going long on the ETF in question.
Ladies and gentlemen, strap in for take off! We're going to Mars. And that's just the beginning.