Oops I've been summoned. So...get this. Space as we commonly know it is three-dimensional, but in fact there are at least five we can directly observe and up to 11 in existence. But for now we're talking about the ones we have directly observed.
We are three-dimensional beings. We have length, width, and depth. This much we know. Well,
Carl Sagan can describe the basic information and introduce you to the concept of time as a spatial dimension better than I can, so I'll let you watch that video.
Now, going to the concept of time as a spatial dimension. If any dimension is a vector, think of what happens when we move an object, even in a single dimension - along a single vector - for instance, a straight vertical drop for a rock into water. The ripples move in ALL dimensions.
Now, imagine the apple moving through Flatland again. Suppose Triangle decides to try taking a bite out of the apple. Imagine Triangle's corner scraping a piece of apple off as it moves by. Of course, the piece of apple is still only going to be a two-dimensional "moment" to Triangle, but let's look at the apple's perspective. As it moved by, a piece of it just got ripped out. Imagine the physical forces in play on the apple. The change is mostly unidirectional, but there's an omnidirectional "shock" that propagates through the entire apple.
So then, suppose time is a spatial dimension that we only see a small piece of as we go. There are, then, three possibilities. One is that it just behaves that much differently than any other dimension and everything we do to it is 100% unidirectional, which then becomes our standard view of the nature of time. But, if time is a spatial dimension it should behave like any other spatial dimension. So, we're left with two equally terrifying possibilities:
One is that everything is fixed - past, present, and future. Our free will, our lives, EVERYTHING about us, is just an illusion as we observe a fixed path in a fixed dimension.
The other is that the past is just as mutable as the future, and in fact EVERYTHING we do sends ripples back in time and changes the past in some small way, and our minds simply adapt to the changes because...the new past is now our past; naturally we remember things as they were as a result of what we've done.