Equivalent Series Resistance (ESR) is the actual ohmic series resistance within a capacitor. When a capacitor charges or discharges, the electrons flowing through the capacitor, in conjunction with the ESR, cause energy loss in the form of heat (like electrical current flowing through a resistor). The lower the ESR, the lower the energy loss converted to heat. Conductive organic polymer, when used to replace the MnO2 (manganese dioxide) in the cathode structure, reduces ESR since the polymer is more conductive than the MnO2.  It also eliminates exothermic ignitions by removing the source of the oxygen and reduces dielectric damage due to “gentler” processing, thus the need for less de-rating. Finally, as ESR is lowered, the capacitance roll off versus frequency (cap loss as you go to higher frequencies) is pushed out to higher frequencies allowing the designer to use less capacitors at frequencies ranging from 10kHz to a few MHz. This is what is depicted in the chart. The higher ESR cap, the T491 series loses capacitance at higher frequencies, while the KEMET Organic T520 series holds on to its capacitance much longer.