Determining voltage and current changes to make components compatible with the voltage source
Any supply, if you change the load on that supply, voltage or current will change.   

Batteries are constant-voltage devices.   Meaning a 9V battery will supply whatever current your load asks for, at around 9 volts. (It may sag slightly).  Almost all supplies are constant-voltage. 

An LED works best as a constant-current load, that is, it is happiest if the supply is limiting its supply current to a specified amount.  This is done by varying the supply voltage, increasing it if current drawn is too little, decreasing if it's too much.  Supplies don't normally do that, so the state of the art for high-power LED lighting is an active driver circuit which does.   

However, for a small job, a resistor will suffice.