The Tesseract is a fourth dimensional cube. As you may know, the 1st dimension is a line, the 2nd dimension adds width to the line (square) , and the 3rd dimension adds depth (cube). The 4th dimension is impossible for us to imagine because we live in a 3D world, but mathematically it exists. In his theory of special relativity, Einstein called the fourth dimension time, but noted that time is inseparable from space.
Imagine how confusing a drawing of a cube would look like to someone who lives in a 2D world and has never experienced a 3D world. To them it would be overlapping squares. That’s exactly how we perceive the 4th dimension. We don’t understand how it looks but we can represent it on a 3D world.
If anyone is interested here are some cool articles on this topic. (Can our brains perceive the 4th dimension?) (The 4th Dimension)