You don't give us quite enough context to go on, but i'm assuming this is some kind of domestic appliance, in which case, it is likely that they are referring to the use of the type of dimmer switch (either installed, or cable-mounting) that is commonly used in domes for regulating lighting.

You can't per se change the 'speed' of electricity, but you can use such devices as these to control how much power reaches the load.

It could be referred to as a 'speed controller', certainly — but if this is a domestic situation, people are more likely to be failiar with is in its dimming function.

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Note added at 1 hr (2008-01-12 14:54:13 GMT)
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Oops, sorry for the typo: "...commonly used in homes..."

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Note added at 2 hrs (2008-01-12 15:21:19 GMT)
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I think Chris is probably right, when he says that it needs to be kept fairly general; the 'lighting dimmer' is cerainyl one possible idea, but by no means the only one, and as Chris says, self-contained speed-controllers do exist on the market, suitable for certain but by no means all kinds of motorized appliances.

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Note added at 3 hrs (2008-01-12 17:00:54 GMT)
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Just for the sake of completeness, I think I need to clarify my position (which I believe is broadly in line with Chris's)

Most modern control devices of this type are continuously-variable (i.e. not operating in switched steps) solid-state devices (some variation on theme of choppers and SCRs / thyristors etc.). This type of device produces a distorted output waveform which may well be satisfactory for operating lamp or heating loads, and even certain types of simple motors, but which can play havoc with the operation of more complex systems, particularly ones that themselves contain active electronics (including electronic 'ballasts' for T/H lighting, etc.)

This is usually why appliance manufacturers warn against using this type of controller on their products.

The much older, and now largely obsolete, purely resistive type of controlller (either a rheostat, or sometimes switch-selected fixed resistors) is less likely to cause problems for appliances of this sort — but is also much less commonly encountered these days, at least in a domestic situation.

And just to be pedantic a bit longer: while some control devices may indeed be specifically used to control current, e.g. the device to control arc-current mentioned in one of BDF's refs., more generally, we would regard these as power controllers — though again, in layman's terms, we more often think about the outcome of that power control, i.e. dimming or speed control (the 2 main groups of applications). Colloquially, people often refer generally to a 'dimmer', particularly when it is installed in a house, even though it is possible some other device might in fact be connected to it...

And finally, do note that over here in France, houses are much more commonly than in the Uk fitted with sockets on a switched lighting circuit — so that you can plug in say a standard lamp and have it come on with the main room light switch. With the incresing use of remote controls and other forms of home automation, dimmer switches throughout are becoming comparatively more common than they might have been even say 10 years ago. Not to mention plug-in remote-controlled dimming devices, etc. ...