My book says,

''consider a ray OA. If this ray revolves about its end point O in an anti-clockwise direction and takes position OB, then we say the angle ∠AOB has been generated as a positive angle and a negative angle. or simply say that angle is a measure or an amount of revolution of a given ray about its initial point.

The angle is positive or negative if the initial line revolves in anti-clockwise or clockwise direction to get the terminal line''

Still, I cannot understand the concept. Could you get the point easier for me, please?

The wording is a little odd; did you quote it word for word?

The basic idea is that anti-clockwise (or counter-clockwise) rotation is considered to be positive, and clockwise rotation is negative.

I would restate your sentence, "The angle is positive

or negative if the initial line revolves in anti-clockwise

or clockwise direction to get the terminal line" as "The angle is positive if the initial line revolves in an anti-clockwise direction to the terminal line, and negative if the initial line revolves in a clockwise direction".

So if we start with OA and rotate 90 degrees clockwise to OB, we say that AOB is -90°.