Line data (of any format, be it GIS/CAD/etc.) typically defaults to the 'direction' of the line being based on start point or first coordinate given to end point or last coordinate given. Direction of travel is therefore considered in terms of with or against the direction of the line.
The most common scheme (there can be others) for attributing this in a network environment (ie, a road system with one-way restrictions) is:
- 0 = travel in both directions
- 1 = travel only with the line direction
- -1 = travel only against the line direction
Note that line direction also affects left and right side of the line in a similar manner. Left is always the left side of the line considering the line direction; the same for right side.
Another related concept is To and From. These are often used in field headers (such as a To address range and a From address range).
- To = with line direction
- From = against line direction
In order to check a line's direction, you should be able to add some indicators via Symbology in whatever software you're using. It's easier or on by default in some software than others, as you see with the OSM editor. If you indicate your software someone might be able to give specific instructions if needed.
It's also possible as mentioned in comments that some lines may become reversed through geoprocessing operations - splitting a line into segments for instance. If you're just saving or exporting data however, that shouldn't happen (which is not to say it can't).