How to find the ESR of a capacitor
If a datasheet just says 'low ESR' without specifying a value, you are usually fine with any style of capacitor with a relatively low ESR. All this really means that you should avoid cheap unrated aluminum electrolytic capacitors, since their ESR is terribly high and can be several ohms.
In this case it wants a 'low-ESR' capacitor for the 1000 µF output capacitor. I don't think I've ever seen a ceramic 1000 µF capacitor and a 1000 µF tantalum capacitorwould probably cost US$50, so you are going to have to track down a low-ESR aluminum capacitor for this application. The output ripple will decrease linearly with the ESR of the capacitor, so lower is better up to whatever price you want to pay.
As an aside, that is a ridiculously high required output capacitance for a switching regulator in that voltage range. You may want to take a look around for a regulator that meets your needs, but is stable without such a requirement. Don't get me wrong, usually the more capacitance the better, but 1000 µf is really high for a 1.5 A power supply.
It's 'equivalent series resistance', and is somewhat frequency dependent. Basically it's the unavoidable ordinary resistance that comes along with the capacitor.
Lower ESR means that the capacitor is more like an ideal circuit element. Resistance just dissipates power, which results in heat, which is generally no good for capacitors, especially electrolytics.
Just speculating now - On that data sheet you linked, the salient parameter looks like 'tangent of loss angle'. If one assumes that 'loss angle' is the angle away from a purely capacitive reactance, then the tangent of that angle would be the series resistance divided by the capacitive reactance, in which case this number being low would imply ESR to be low.
How "low" depends on the efficiency and reliability you are trying to achieve. For low ESR capacitors the manufacturer will supply the values.
Search for Nichicon low ESR capacitors and you will find parts that have a low ESR. The VR series is not a low series resistance capacitor. The PM Series is and the ESR is specified in the datasheet. Nichicon (which makes excellent capacitors) may have some newer series.
The ESR is critical to the life of the capacitor since as ESR increases the temperature of the capacitor will increase which will decrease its life.