DPI (dots per square inch)
Images are made up of dots of color. The more dots you pack into a square inch the higher the resolution. High resolution files are always what you are looking for in printing, but sometimes there has to be a trade off since the higher the resolution the larger the file becomes.
For banner printing a max of 125dpi is suggested, same for large fabric displays.
Designing at half or quarter size?
If you target resolution is 100 dpi at actual print size you can design at half size with 200dpi images or at 1/4 size with 400 dpi images.