(Massive edit to indicate ChrisF's alternate reading and highlight why that sort of thing makes metaphors a bit rubbish).
No idea where it came from but in terms of what it could mean:
Resistors in series (that is one after the other) essentially combine together as the sum of their values. So one 100 ohm resistor and one 200 ohm resistor in series are the same as one 300 ohm resistor.
In parallel it's different - they share the burden and the total is actually less than any of the individual values. So if you have 100 ohm and 200 ohm resistors in parallel, their equivalent value is 66.66 ohms (1/R = 1/R1 + 1/R2).
So as to what that means in terms of programmers, there are two ways to think of it:
1) Resistance is an indicator of resistance to work getting done (that is the lower the resistance the more work gets done):
If you have one good programmer and one bad programmer, even if the bad programmer only does a tiny bit of useful work, that's a bit of work the good programmer doesn't have to do, hence overall they are more effective together than he would be on his own.
Obviously there are all sorts of issues that can be raised with this - that bad programmers can actually be negatively effective, that if you follow this through to it's (il)logical conclusion means that throwing people at projects makes them faster and so on.
2) Resistance is an metaphor for productivity (that is the higher the resistance the better):
If you read it this way then every extra developer actually reduces the total productivity below that of the level of the weakest member, essentially saying that they're all slowed down by the required co-ordination and communication.
Again, lots of issues (primarily that we've all seen teams work) but in it's own way potentially a fair point.
So the overall point would be that it's a bit of a rubbish metaphor as it isn't clear and has two possible readings, with completely opposite meanings. Welcome to why metaphors, while useful, are often evil.
(And for the record that's the first time I've used my degree - Electronic Engineering - since graduating over 16 years ago. This is year 1, term 1 stuff and I still needed Google to nudge my memory and have probably still got it wrong).