The first dimension is a line. Every one dimensional thing is a line, and the only distinguishing feature is the length.
The first dimension, at first glance, seems pretty pointless. But their are some benefits. One of the benefits is that 1D strings could be used to connect 3D objects. Since 1D strings are infinitely thin, they could pass through anything.
If they where somehow connected to two objects, the objects would stay together without having any visible or tangible connection. As you can doubtlessly imagine, this would have tons of benefits in everyday life and industry.
The following is an example of a 1D line:
Note: ignore the thickness of the line, it is there because it is very hard to see something infinitely small. Be thankful.
No one knows how to connect 1D lines to things. Also, no one knows how to make 1D lines.