Notice that there are three (3) pins on a 30 amp plug. These are "ground", "neutral" and "hot". The electricity is supplied through the "neutral" and "hot". "Ground" is used for protection of down-stream circuits. In fact, any current in the "ground" line (or unequal currents in the "neutral" and "hot" lines) is considered a "fault"; a GFI (or GFCI, "ground-fault circuit interrupter") outlet senses this and disconnects the down-stream circuits.
50 amp service is more like the electricity that is supplied to most homes: 2-phase, 240/120 volt.
Notice that there are four (4) pins on a 50 amp plug. These are "ground", "neutral", "phase-1" and "phase-2". "Phase-1" and "phase-2" are both "hot", but they are opposite in phase. This means that each of them can supply 120 volts relative to "neutral", but 240 volts relative to each other. Thus a 240 volt appliance would be connected across the two phases, whereas a 120 volt appliance would be connected across one of the phases and "neutral". A 50-to-30 amp adapter simply passes the "ground", "neutral" and ONE of the phases through from the 50 amp side (plug) to the 30 amp side (socket).
As a side note, I don't know whether these adapters all choose the same phase to pass through, or whether there is some agreement among manufacturers to (perhaps randomly) assign different phases to different batches. If they all chose the same phase, this would put a terrible imbalance on the electrical system of the RV park; all of the 30 amp rigs would be on the same phase. Maybe someone else can shed some light on this for me. Maybe the RV parks balance the load on the phases by wiring alternate pedestals with reversed phases, or some-such.
Also, note that there is a BIG difference in the two systems' ability to supply power: a 30 amp source can provide up to 3600 watts of power (120 x 30), whereas a 50 amp source can supply up to 12000 watts of power (240 x 50).
