The idea is to use left and right pedals (brake/back, and accelerator) to use a succession of trucks to pick up minerals from your depot and then drive over a challenging 2D landscape, desperately trying not to dislodge your load, to the destination depot, where the minerals are unloaded and coins collected as a reward.
Gameplay is very smooth, with parallax scrolling of backgrounds, realistic physics as the truck bumps over rocks and wooden platforms and hills, rising and falling on its suspension and with your precious minerals jolting around and (sometimes) falling off completely.
You also have to manage backing up to receive your cargo accurately, plus you have to stop before hitting the wall in your final depot, else you'll either not be carrying a full load or will lose some of it at the end, in each case losing more coin-earning ability.
As you might expect, more coins eventually means buying new trucks, which can hold more minerals and thus you can earn more, plus you can buy new landscapes as well. But don't worry, since the coins concerned are well balanced and this isn't one of those freemium titles designed to trick you into shelling out a fortune.
Typically, here, you'd try the game for a few 'maps'/levels, decide you like it but that earning the next truck and landscape is going to take too long, fork out for the second in-app purchase of coins (£1.29p in the UK), which gets you both (and then some), and then you're off and running.
Of course, a slightly greater purchase (up to £2.29 in the UK) gets you all the coins you could ever want and (almost) all the trucks, plus if you play for many hours then you'll empty the 'fuel' can and need a re-fill, which is another £0.79. But the value per hour of gameplay is still pretty high, especially compared to some other freemium culprits on mobile platforms. The developers have pitched freemium nicely here, I think.