The tester will go through all the wires in sequence. If the sequence of the green lights blinking does not line up you might have swapped some wires when wiring the connector.

If some lights do not light up there might be a break in the cable or you made a mistake when crimping the connector.

For Fast Ethernet (100 Mbit/s) you only need pins 1, 2, 3 and 6. Some cheap cables may not even have more than these four wires.

G means ground. For most consumer applications up to 1GbE it should be fine if your ground is not properly connected.