I don't know how other people determine what it means for them to understand something, but IMO if you talk in abstract space-time bending (as most science communicators do) instead of river model, you don't understand relativity - you just explain math and repeat what you've memorized.
https://www.youtube.com/watch?v=DjwQsKMh2v8
Most YT channels cannot explain relativity and instead regurgitate diagrams or abstractions directly derived from math equations, talking about "spacetime bending" like everyone knew what it means.
It's nice to see something concrete breaking through that noise from time to time: https://www.youtube.com/watch?v=JFEMgaAusic
GPS satellite clocks, despite their orbital motion, run faster due to low gravity on high orbit.
Δt = 7 - 45 = -38 µs/day
OSS clocks on the other hand run slower, because it moves 2x faster on much lower orbit (you need to move faster to stay on lower orbit).
Δt = 28 - 3 = 25 µs/day
Astronauts on OSS age a tiny bit less than we do here on Earth. It's not like they gain some extra time there - they also "live" less time than we might think they are.
Spacetime moving in a gravity field is enough to cause time dilation, but without acceleration it wouldn't be able to keep you on the ground. In inertial (constant speed) reference frame you'd be able to jump into outer space.
Gravity can't pull you down directly, but due to "vertical" acceleration of space some parts of your body would move slower (from surface) than others, so in a way you pull down yourself.
The source of time dilation in General Relativity is the same as in Special Relativity. The difference is that in a gravity field you're inclined to measure speed relative to the mass in the center (a planet, black hole...) instead of space itself.
Space-time fields do bend, but that's just math - visualized coordinate system. When you stand still on the surface of the Earth space moves through you.
Contrary to popular belief, faster-than-light travel wouldn't move you back in time. Instead, assuming movement as we know it and ignoring other effects, after reaching your destination you would be able to see two versions of yourself from the past - one preparing to start and the other moving backwards in a spaceship. Both would meet sometime after the start and disappear.
From breadcrumbs collected so far we have:
(1) Physical fields of space
(2) through which light travels at constant speed and nothing can move faster,
(3) but in a moving frame of reference photons bounce back from mirrors and come back at the same time regardless of direction.
Animation below shows that it would work if lengths were contracted in the direction of motion, but we assumed they're equal. How do we measure distance anyway?
This is simplified version of expected result from Michelson-Morley experiment. We know that this is not what happened and photons came back (to A) at the same time.
Note that photons didin't move at the same speed relative to the central object A (see previous entry). That's not how Special Theory of Relativity (STR) solved this problem though.
Couldn't comprehend statements like "Speed of light is constant and the same for all observers regardless of their motion and direction they measure it" and felt like you're too stupid to understand Theory of relativity?
Well, it's an example of misleading simplification that sounds fascinating, but notice how it always kind of ends with implicit "trust me bro".
Here's a video that should trigger "wait a second" reaction: https://www.youtube.com/watch?v=pTn6Ewhb27k
Physics students learn the speed of light, c, is the same for all inertial observers but no one has ever actually measured it in one direction. Thanks to Kiw...
shudder 