How can time be manipulated when it is only a measurement?

I’m no scientist by any means, so please – keep it in terms I can understand, haha.

I can’t wrap my head around “freezing time” or “going back in time”. My very basic understanding is that time is a measurement – a tool created by us humans to know when to plant seeds and prepare for the winter; without humans, time (and our perception of it) would not exist.

My reason for asking is that I recently heard of an experiment where scientists took two atomic clocks, synced them, left one in a controlled environment and put the other on a supersonic jet.

The results showed that the atomic clock (after flying around for a while) on the plane was off in comparison to that of the controlled-enviro one.

I understand that, in theory, time is relative to speed – in crude terms, if you go faster, your personal time is slower than everyone else. I just don’t get why.

So, anyone out there willing to help out?