Single axis tracking simply means there is one axis of rotation. The axis can be horizontal (most common), tilted, or even vertical.
A horizontal single axis tracker is the most common configuration. The axis of rotation is horizontal, usually orientated North-South with the modules facing toward the East in the morning and the West in the afternoon. It is common for the maximum allowed angle to be 45 degrees from horizontal. Thus in the early morning and late afternoon the tracker does not move and remains at a 45° tilt angle. Some tracking systems use “backtracking” to avoid row-to-row shading. This means that in the beginning and end of the day, the tracker reverses direction.
Here is a single axis tracking PV system near Santa Fe, New Mexico.