The fourth dimension is nearly synonymous with science fiction and futuristic technology, but not only is it a part of our reality, it also holds the simple explanation of time. Granted, the fourth dimension can be classified in several different ways. For example, fourth-dimensional constructs of all geometric shapes can be made in which all four dimensions are solid, tangible planes. In physics however, the idea that time is its own dimension is useful, as it affects every aspect of our physical world.
A dimension can also be defined simply as a direction. A one-dimensional shape is called a line. It is the only shape an object with one dimension can take, because there is only one direction available, length. A two-dimensional object has a greater variety of forms because it has two directions, length and width, that it can occupy. These encompass the basic shapes of circles, squares and triangles that we first learned as children.
In three-dimensional space, which is what we humans move through, length, width and depth are all available to us. However, human perception is limited; a human mind will never be able to fully comprehend more than three dimensions at once. We can only see brief parts of this fourth dimension, time.
Consider a cube. It is an object in three dimensions, but it is made up of two-dimensional squares. Slicing thinly enough, this cube could be cut into a near infinite amount of squares, each separate from the others. To a two-dimensional being who could perceive no more than length and width, only one of these squares could exist at a time, and while this being may be able to move from one square to another, they could never exist in more than one square at the same time. So it is with time.
To us three-dimensional beings, time is a series of thinly sliced moments, one progressing to the other in sequence. We can only perceive the moment that we are currently in. The future is hidden from us, and the past is only preserved in our memories. In fact, this fourth-dimension is so closed to us that we can only move in one direction across it. Time is indeed an alien dimension, and we will never be able to grasp enough to understand it.
To simplify the idea of the fourth dimension as time still further, suppose two friends wish to meet, they can say they will meet on the Empire State Building in New York. When you consider the entire world, one building is a small enough space. However, the Empire State Building is over a hundred stories tall. They could look all day and still pass by each other. So one might suggest they meet on the roof. This would make their meeting much more likely, but one more parameter is still needed. If these friends were to agree to be on top of the building at noon on a certain day, they would be combining length, width, height and time, having one exact point at which to meet.
Although we do not understand time, it moves through us and around us, and we as humans can only catch oblique glimpses of it as we pass.