This is a tube-era resistor/capacitor subsitution box, which makes it useful for... tube circuits. For substitution boxes, the "restoration" work to be done is mainly testing the components to see whether they are close enough to the state value. The tolerances are terrible (so bad that I decided to relabel some positions rather than replace components). But the capacitor voltages are high, and the resistor wattages are high (compared to typical solid state stuff). The unit I have was missing an alligator clip, and I reformed the two electrolytic capacitors. So far, I've found it useful primarily for testing whether power supply capacitors are doing their job - you can clip this in parallel to see if the extra capacitance improves the ripple.
There are plenty of articles with better detail, background, and instructions on reforming, but here's the gist. Electrolytics have an oxide layer that provides the dielectric barrier between the plates of the cap. The oxide layer is actually created and maintained by the application of voltage. Having an electrolytic sit unused for a long time can result in this layer breaking down gradually (and applying a reverse voltage makes it break down REALLY fast - that's why these things are polarized). In practice, this means that a cap rated for 450 volts might not have a good enough oxide layer to insulate that high a voltage, and it will short out and explode if you just connect it to the max voltage. This is why many sites will warn you against just plugging in and turning on old equipment that's been sitting in a closet for decades.
Side note: personally, I haven't had too much bad luck with this, although I try to first test suspect equipment with a series lamp circuit (need link). I think (and am too lazy to verify) that a tube rectifier will produce an output voltage that rises as the tube warms up, giving the subsequent capacitors a fighting chance (in essence, I suppose, a very brief reforming period). I'd be more cautious with a diode rectifier, which would immediately deliver the full voltage to the caps. The lamp limiter is nice and easy. You also see recommendations to bring things up on a variac. I have one but have never hooked it up. Guess I'm a little scared :)
A capacitor that needs reforming might measure more or less ok. I tested a few capacitors before and after reforming, and capacitance, ESR, and Vloss didn't dramatically improve. In fact, they might go up or down a bit. It would appear that the only purpose (though a worthy one) of reforming is to ensure the capacitor will perform up to its rated voltage.
So what is it? Basically, you put a largeish resistor in series with the cap and connect them to the cap's rated voltage. Monitor the voltage drop across the resistor, and you will hopefully see it drop over time. The resistor serves to limit the current into the cap, which should prevent catastrophic failure of the cap and will hopefully regrow the oxide layer. What value of resistor should you use? I've seen 100R (too low) to 470K (too high for someone as impatient as me). I'm using about 30K, because it's what I had. Very important - you need to make sure the power handling of the resistor is high enough to handle the full voltage. At the instant of turn on, it WILL handle the full voltage, because the capacitor looks like a short at the very beginning of the process. If the capacitor shorts out, the resistor will also be taking the full voltage. Calculation time. If I want to reform a 450v capacitor, then my 30K resistor would have 450V/30k=15mA of current through it. At 450V, that's 6.75 watts. I've got 4 5-watt resistors in series to make my 30k resistor, so I'm good. I DID use a lower-power resistor first, and it DID smoke a little. Don't do that.
How long do you let this go? Well, one rule of thumb is 5 minutes plus 1 minute per month of storage. Which is a long time for ancient capacitors. So it might be 12 hours, or even overnight - not that I recommend leaving this process unattended. Especially keep a close eye on it for 5 minutes or so - the voltage across the resistor should drop substantially at first. Or the cap might fail. Not all caps can be saved - if the voltage across the resistor won't drop below 10% of the rated voltage after an hour, you should probably discharge the cap and chuck it. Eventually, the voltage drop across the resistor will be pretty darn small and relatively stable. At that point, hooray, it's reformed and you can use it!