A blocking diode allows the flow of current from a solar panel to the battery but prevents/blocks the flow of current from the battery to solar panel thereby preventing the battery from discharging.
Current streams from high to low voltage, so on a bright day, the voltage of a panelboard will be higher than the voltage of a profound cycle battery and this energy will normally spill out of the PV panel to the battery. However, at night, if the panel board is associated specifically to a battery, the voltage of the panel board will be lower than the voltage of the battery, so there is a probability of some retrogressive stream
To prevent this from happening, a blocking diode is installed. It allows the current to flow from the panel to the battery but blocks the flow in the opposite direction.
It is always installed in series with the solar panel.
Blocking diodes are not in the typical PV system these days, however, in the past they were popular. They prevent reverse current. In simple terms, you could refer to it as a one way street.