You have probably seen a seven segment display. It is a device (sometimes electronic, sometimes mechanical) designed to display a decimal digit using a subset of seven segments. If you still don't know what it is, check out the following link in the Wikipedia article.
Program that can simulate the operation of a seven segment display, although you are going to use individual LEDs instead of segments.
Each digit is built with 13 LEDs (some on, some off, of course), this is how we imagine it: