The two identical items on the left are ignition protected fuses (one for each battery) that are made for use around gas fumes. The red box is nothing more than a switch that turns the system on and off but also allows the two batteries to be combined for additional starting current. The black box next to it is the brains of the system. When the motor is running and the alternator is working the black box connects the two batteries so that both the starting battery and the house (electronics) battery are charged. When the motor is off, the black box opens so that the electronic/lights/bilge pump/etc. are running only off of the house battery so there is no load on the starting battery.