A phenomenon that, although appearing occasionally in the most remote parts or on the least significant roads in England and Wales, a single track road with passing places is very much a Scottish thing.
In the Highlands, many roads – even fairly major roads, and in the most remote North, even the main trunk A-Roads – are single track.
A single track road is a road that has only enough room for one vehicle. Usually a vehicle means a lorry, which means that in SOME places two smallish cars may be able to pass slowly, but in the vast majority of cases it's the passing places that must be used for that purpose.
Passing places are wider areas of the road sort of bulging out on one side, usually enough for a lorry or two-three normal cars to fit in.
There isn't really much mystery or difficulty in using single track roads, although they can be tiring because they require constantly paying attention even if there is very little traffic.
The basic principle is simple: the cars need to pass in the passing places, and if you don't notice the car approaching soon enough, one of you would need to reverse. Always stop at the correct side of the road – ie when the actual widening in the road is on the left, you should still stop on the right and the car approaching will pass around you using the space. Usually people tend to wait in the passing spaces that are on their side of the road, as it feels more natural than stopping in what feels like the middle of the lane.
Passing spaces are also used to allow overtaking. Please, please, please do that if driving in Scotland. On country roads, especially the wilder ones, the very local locals who know each bend and pothole drive much faster than the visitors, and even the less-local locals going about their daily business tend to drive faster than those who look at landscape and amble about. If you see somebody catching up behind you, pull in into a passing space and let them overtake.
Most single-track roads have plenty of passing spaces, but on a winding or a very hilly road, it isn't always possible to see to the next passing place and thus some degree of planning is required.
Essentially, though, just being watchful and taking your time is all you need.
And remember, never, ever use a passing place to park! Stopping to check a map or something similar is OK (use the indicator to encourage overtaking) but leaving your car is a real no-no.