An ideal voltage source is an abstract thing that provides constant voltage source
no matter what.
A battery can be represented by an ideal voltage source (lets call it Vs) in series with a resistor (lets call it Rs).
When the battery is not being used (no current flowing), the circuit is said to be
open.
When you apply some other resistive component Rl to the battery, we say that the battery is
under load.
The voltage accross Rl is given by Vs*Rl/(Rs+Rl). You can see that when Rs is much smaller than Rl, the voltage approaches Vs, i.e. the open-circuit voltage. On the other hand, if the load resistance is, for example, equal to Rs, the voltage is just Vs/2.
That's why we want the internal resistance Rs to be as small as possible (as with any output resistance for that matter), and the input resistance of a device to be as high as possible.
I suppose a professional tester will tell you the Rs, Vs, and maybe some other auxiliary stuff.
P.S. is
this professional enough for you? Damn I want that toy!