Light emitting diodes, "LEDs" convert electrical energy into light (and heat). When enough voltage is applied, current(electrons) begins to flow through the diode. In general, thee more current that is flowing through an LED, the brighter it will be. LED's have a limit to how much current they can handle before they will be destroyed. Even within the limit, the lifetime of an LED can be dramatically shortened, if its current is higher than recommended values.
The properties of LED's (and most diodes) are such that they offer very little resistance to current, and so they will accept as much current as is available until they self-destruct. This is why you typically need a current-limiting resistor connected in series with the LEDs. A resistor has a linear relationship to current and voltage, so it prevents "runaway" current levels form passing through the circuit.
All circuits are government by "Ohm's Law", which
states:
V = I * R
where:
If you know any 2 of those values for a component, you can calculate the other. A standard resistor component has a fixed, constant value for R, so the relationship of voltage to current through a resistor is linear. LED's do not have a fixed resistance, it varies with current and voltage. We often treat LED's as ideal diodes, which means that below it's "bias voltage", it allows no current through it and above the bias voltage, it allows "infinite" current. This is a pretty good approximation (except that real LED's will self destruct with too much current).
There are 3 modes in the tool, for setting which part of Ohm's law you want to solve. Usually, you have an existing power supply/battery, and the LED's have a specified maximum current, so you want to find the ideal resistor value to achieve that current.
Select your mode form the mode-drop down list to determine what value you are solving for, supply voltage, resistor value, or total current. Tap or click on the components to change their values. Components with green labels are inputs and can be edited. Components with white labels are the outputs and are calculated by the tool.
You may also add and remove LED's in series in the circuit. The bias voltage of LEDs in series adds together. If the total bias voltage of the LEDs exceeds your supply voltage. No current will flow, and the tool will warn you as such.