The ui.ini file is used to store the settings of the internal user interface used by MAME. You can find the file in C:\Mame folder. This is where MAME stores colour, layout, image and path settings and it can be edited to customize how the user interface is displayed.
To create the default ui.ini file use either of the following command lines...
mame64 -createconfig
mame64 -cc
To edit the file in Windows, find the ui.ini file in the C:\Mame folder. To open the file you may need to associate it with a text editor. To do this, simply right click it and select the "Open With..." option. Use Notepad if you don't have your own prefered editor. You can now double click the ui.ini file and it will open.
Here is an example of a ui.ini file...
As with the mame.ini file, the options are grouped together under headings. The format of the options are fairly simple to follow. Most things with a yes / no answer are denoted by 0 for no and 1 for yes. there are also a few numerical options (e.g. gamma) and a few that require a text answer (e.g. rom path)
Finally, If you mess the whole file up, just delete the ui.ini file and run mame64 -cc from the command line to make another.