The two main voltages used in Eurorack are -12V and 12V. So there are typically three wires going from the power supply to the busboard, supplying -12V, 0V (ground), and +12V. The +12V is 12 volts higher than 0V (ground), and 0V (ground) is 12 volts higher than -12V. In many amplifier designs (and almost all modules have some sort of amplifier), it is convenient to have both positive 12V and negative 12V supplied.
+5V is not used by as many modules. +5V was a standard for digital electronics, so it tends to be digital modules that require it.
If you need +5V, some busboards must get it from the power supply (so they need both: -12V, 0V, +5V, and +12V). Other busboards can generate +5V themselves from the -12V, 0V, and +12V voltages.
No modules need -5V, and this voltage is never used.
When you connect modules, it is very very very important to line up the ribbon, so -12V on the module connects with -12V on the busbuard. Getting it wrong can destroy modules without reverse polarity protection.