If by "lights" you mean incandescent lamps, you can connect them in parallel to a 1.5 volt source (like a D cell). In this mode, there is no particular reason that all lamps must pass similar currents, only that they all require the same voltage.
If they all use the same amount of current they can also be connected in a series string and the string connected across a voltage supply that is 1.5 volts times the number of lamps. In this mode, there is no particular need for them to each require the same voltage. E.g. 4 1.5 volt lamps in series would need a 6 volt supply but would operate correctly only if they were all designed to operate at the same current.
The rule is that voltages in series add (in this case, the voltage drops of each of the lamps). Currents in parallel add, so connecting the lamps in parallel does not change the voltage requirement, but the current the supply must provide is the sum of each of the lamp currents.