The concept is that time is another dimension, complementary to those we can observe and measure directly. For those three, I can take a ruler and measure how many millimeters one point in space is far from another. If time is another dimension, and if I had the ability to observe that dimension as I observe the other three, I should be able to take my ruler and measure a distance in millimeters along that dimension as well.
This leads me to ask the question: How many millimeters are in a second? As in, if I take my ruler and measure between a point located in a 3D space at a particular time, measure along the time axis of spacetime to the same point in 3D space one second later (or earlier), I should be able to come up with a conversion ratio between millimeters and seconds, right?
If this is so, would there be a meaning to a question like How fast are we moving through spacetime, along the time dimension? As in, if we are following a vector in spacetime, what is the meaning of the magnitude of the time component of that vector?