Maybe a stupid question but what does this mean?

Reimann said:

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My girlfriend's car had an oil leak and this is the paperwork she got back with it. I don't know what a sending unit or switch is. A pump of some sort? Anyway, I was hoping someone could explain this for me.

And since I know I am going to get flamed for asking that, yes the car's blinker fuild is filled and the fan belt buckle is secure and the muffler ball bearings are well greased...

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The sending unit detects your engines oil pressure and sends a signal to your oil pressure gauge.

buuuut it shouldnt be causing an oil leak unless it was where it attached to the block

Most oil pressure sending units screw right into an oil gallery, usually down near the filter and consist of a chamber separated into two sections by a rubber diaphram.

One side of the chamber is exposed to the oil gallery.

The other side of the chamber has a device that measures displacement of the diaphram, like a linear potentiometer, along with a spring.

When the oil pressure builds, the oil-side section of the chamber sees oil pressure which forces the diaphram to move. This compresses the spring in the measuring side and the amount the diaphram moves against this calibrated spring is measured by the electricals. The amount it moves is directly proportional to the pressure in the oil gallery. You instrument cluster (more or less...many are dumbed down nowadays) shows the results of that measurement.

If the diaphram breaks because the rubber failed, the engine oil will get past the hole and into the measuring side, which isn't hermetically sealed, meaning you'll get oil leaking out of it.

Not a terribly uncommon failure.

I've seen a couple of oil sending units that leak, most of the time if not all the time they are a little sensor right by the oil filter. On our Mustangs they are right below the filter.