The battery meter is an ammeter. It will have a 0 amps centered position and + amps and - amps positions.
An indication in the + side of the ammeter shows if current is flowing from the alternator to the battery.
An indication in the - shows excess current flowing from the battery to the accessories or the alternator is not working.
The ammeter measures voltage drop across a shunt. In this case the shunt is the heavy red wire connecting the battery to the alternator and accessories.
The smaller black and blk/wt wires go to the ammeter. If they are not fused they should be.
Inline fuses (4 amp) should be installed in each of these wires at a point as close as possible where it connects to the heavy red wire.