It can only measure or respond to motion in one direction. Think of a single-axis gyroscope like a seesaw that only goes back and forth.
The specific number and orientation of these axes can vary depending on the gyroscope's design and intended application.
While single-axis gyroscopes have just one primary axis of rotation, others, like three-axis gyroscopes, are designed to measure motion and orientation changes along all three spatial axes: X, Y and Z.