An epiphenomenon is something which is caused, but can't itself cause anything. (In this way, the analogy of steam coming off a plate of hot spaghetti is misleading, because steam certainly can cause things -- think of a steam engine.)

The term "epiphenomenalism" is used to describe a view in philosophy of mind -- the view that mental states are epiphenomena of physical states.

Epiphenomenalists believe that mental phenomena (such as sensory experiences, or beliefs) are caused by physical phenomena (brain states) but can't have any effect on the world. I.e., mental phenomena are causally-inert "by-products" of physical occurrences.

In general, accusing a philosopher of making mental states epiphenomenal is a serious criticism -- for it would be a surprising view which held that our desires and intentions don't make us do things, or that my pain didn't make me yelp. There are philosophers who embrace this conclusion, though. (Two quickie examples: functionalists, for the most part, believe that qualia are epiphenomenal. Donald Davidson's anomalous monism may lead to epiphenomenalist consequences.)

Notice that saying mental states are epiphenomenal is different from saying there aren't any mental states (as, for example, eliminative materialists do). It's also different from Cartesian interactionist dualism (epiphenomenalism can be dualist, but the interaction isn't two-way -- physical causes mental, but mental never causes anything).