The difference here is how speed and velocity differ. Speed is a scalar, while velocity is a vector. Speed tells us how fast something is going, but it doesn’t provide information about where it’s going; velocity provides both. For example:
A car is traveling 30 mph. (Speed)
A car is traveling 30 mph northbound (velocity).
In the case of the moon, the moon travels around the earth in a circular orbit, being held in rotation by gravitational and centripetal forces. While the speed remains more or less constant, the direction vector points parallel to the path in which the moon travels (the tangent vector), which changes dramatically as the moon makes a rotation around the earth. This is why the moon goes at an approximately constant speed but not velocity. If the moon went at a constant velocity, it would immediately fly out of orbit around the earth and off into space, and the reason it doesn’t do this is because of the gravitational/centripetal force balance mentioned earlier.
Hope this helps!