mkBiB is an editor and viewer of BiBTeX file. The left hand pane is the main editor, and the right hand pane is the main viewer.
This part is used to create a single bibtex entry giving its value one by one. When finished, press Create button to create a single bibtex entry.
An entry can be generated using Alt+E or Edit ▸ Edit which will open a empty editor to input a complete entry.