While this isn't quite what a "time crystal" is, imagine a two-dimensional universe that contains just one arrow in it. Now spin the arrow. Now plot the full state of the system, using the third dimension as time. You get a sort of spiral thing going along the dimension you used for time. This periodic structure is sort of like what they mean by "time crystal"; it's a periodic structure in the time dimension.

Now, there's more to it than that. I'm not proposing this as a full explanation of what it means, just trying to get the B-class sci-fi ideas out of the way to sketch out the complicated bit of what they mean. As a crystal is a periodic atomic structure in space, a time crystal is a periodic atomic structure in time. There's more to both, of course, but that should help get the woo-woo out of the way.