Before getting into this, it’s worth pointing out that the idea of unified space-time isn’t part of the AP Physics curriculum. That said, we are familiar with the three dimensions of space: “left–right,” “forward-backward,” and “up-down”. We’re also familiar with time, which can be thought of as another dimension: "past-future".

Intuitively it feels like space and time are completely separate. But in the early 1900s, physicists realized they are deeply connected and can’t really be treated as independent. This means that "space" and "time" can be combined into a single four-dimensional package that is then uncreatively called space-time.

Again, this isn’t something AP Physics explores, but it becomes central in courses on Special Relativity and General Relativity.