The upper LEDs from left to right indicate incorrect polarity, a partially charged battery and a fully charged battery. In this case, a fully charged battery is indicated by the LED.
The lower LEDs from left to right indicate "Power On", 0.8 A at 14.4 V, 3.3 A at 14.4 V and 3.3A at 14.7 V, the latter for specialty batteries and charging in colder locations where a higher battery charge voltage is required. The LED indicates that the charger is in the 14.4 V - 3.3 A charging set-up.
If I have any complaint about this or any battery charger / maintainer, it is the low quality of the wiring which is too stiff even in temperatures above freezing (10 C for example). Anything below 0 C, good luck trying to get the cable to lie flat.