Einstein (apparently) described it as that thing that keeps everything from happening at once. But, that doesn't help.
Length is perpendicular to width. Height is perpendicular to both, simultaneously. Carl Sagan described the 4th dimension (which time is supposed to be) as the dimension that is at right angles to these first three -- simultaneously. Try as you might, you will not be able to picture that. He tried to help us understand this by describing Flatland. Look that up if you aren't familiar with it. It's pretty interesting.
How this ties in with time though, I am not sure. But, someone on YouTube tried to visualize time. He showed a continuous walk cycle of a person--like film frames almost. It was a long, frozen, series of many separate movements, each slightly separated and moved a bit from the one before it. This almost seems like the 4-dimensional thing Sagan was trying to describe (maybe the past and future movements were somehow at right angles to the first three spacial dimensions?!?), but the guy narrating the video did not say this, if memory serves.
Maybe the fourth dimension is only a spacial dimension and time is something entirely different. I am not qualified to say in the least.
Anyway, I love thinking about stuff like this, but really, maybe only a handful of mathematicians can understand it -- or so they claim. That it is such a mind-boggling thing shows the mind of our Creator. Holy moly. "Big" is woefully inadequate, maybe even insulting.