Actually, there are three if you count the Auto-hold, EPB and Park. But other than that, what have MG ever done for parking.... (Sorry. It was either that or a Spanish Inquisition gag...)
So. The big knob in the middle is effectively your gear selector like your previous car. Twist for reverse, drive and neutral and press for park. The car isn't going anywhere in park unless you put it back into a gear with that knob. putting the car back into 'off' with the stop/start button will also automatically put the car back into P.
When in park, The P lights up on the knob, along with a P on the dash where it shows the current gear (bottom middle) and a red (P) near the top left to show that the Electronic Park Brake has also engaged.
Behind that you have the EPB (Electronic Park Brake) switch (a rocker that goes up or down.) Up to engage, down (with the brake pressed) to disengage (yeah, you probably already knew that!). This is not park though. If you press the accelerator (or did we decide on Power Demand Pedal?!) then the EPB will automatically disengage and off you go. A red (P) will show on the dash when this is engaged.
Finally, there is the Auto hold button behind the EPB switch. Press to turn on, press again to turn off. When this is on, the car detects when you brake to a stop. Assuming that you have the brake pressed far enough to hold the car still, it automatically hold the brake pedal down for you, and show a green (P) on the dash. If you're there for a while, it'll engage the EPB and release the brake pedal and the (P) will change to red. Either way, you just press the accelerator pedal and it'll release whichever brake is currently applied and go on it's way...
Note that Hill Hold is active even when Auto Hold is off. Pretty similar though, in that if you stop on a hill it'll automatically hold the brake on (but only for a couple of seconds) whilst you move your foot from the brake to the accelerator.
*some features model dependent