esr means
ESR means Electrical Series Resistance of the load capacitor at LDO's output. Typically LDOs need the load capacitors to have low ESR.
To have more insight to ESR effect on LDO performance you might want to read the LDO book written by Rincon Mora (download from this link)
ESR of any device is described as:
ESR is the real resistive component of the complex impedance Z(w) = R + j X(w) of the device; this complex impedance can involve several relatively minor resistances, inductances and capacitances. These small deviations from the ideal behavior of the device can become significant when it is operating under certain conditions, i.e. high frequency, high current, or temperature extremes. Different ways have been developed to represent the non-ideal behavior of electrical components, ESR being just one of them and each tailored to the typical way a component is used.
Good luck.