Steaming in this case means the heat is turned off and the residual steam left inside the pot gently finishes cooking the rice. It helps the grains absorb any leftover water evenly, resulting in fluffy, plump perfect rice. An entry level rice cooker only detects a change in temperature once the water is cooked off causing the temperature to rise so declares the rise ready and switches over into keeping warm mode without the steaming process. The fancier models have various sensors build in for humidity and temperature and a micom (micro computer) build in.

When you look at the "proper way" to cook rice on the stove, it also involves a steaming period with the heat switched off and the lid kept on.

The fancy rice cooker still switches over into keeping warm mode at the end of it where it can remain for up to 13h. Some also contain an actual steaming basket for rice and vegetables. I have a mini Zojirushi which doesn't but the bigger models do.

As for the amount of water used. There is a measuring cup included. Say you fill it up to the measure of 1 cup. The inside pot of the rice cooker has measuring lines for the different types of rice (white, brown, sushi, sometimes others) and you fill up the water to the line of 1.

Hope I don't sound like a total rice nerd but someone asked what the differences where to more expensive models. I appreciate that most are perfectly happy with cheap models and that's totally fair!