Positive terminal of the upper comparator of 555 is connected with 13.8V in order to turn OFF the charger if the battery charges above 13.8V.
13.8V is obtained by connecting a 13.8V zener in series with a resistor.
If the battery voltage is greater than 13.8V, comparator output goes high and flip flop will be set. This turns OFF the transistor and the relay. For understanding relay working see How to connect relay: Relays working with animation