There are 2 different types of resistor ladders, first being an R–2R. This ladder is a simple and inexpensive way to perform digital-to-analog (DAC) conversion, using repetitive arrangements of precise resistor networks in a ladder-like configuration.
The most common type is called a String Resistor Ladder. It's a string of many, often equally dimensioned, resistors connected between two reference voltages. The image shows a common configuration. The resistors act as voltage dividers between the referenced voltages. This circuit is the choice of many hobbyists that run out of IO pins on the microcontroller. This will allow multiple switches or buttons on one pin. Then you write the software to calculate the input voltage, then simply use this data to execute other software.