Sorry ,Please include this fig for explanation.
In second graph , it shows a angular dissplacement of 90 degree from Y-axis to X-axis iyf y axis considered as initial line and movement is clockwise direction.
Displacement is vector quantity and defined as change in
position.A point on a wheel rotates from 12 o’clock to 3 o’clock, so its angular displacement is 90 degrees.You can understand it as let us have a unit circle in xy plane with centre at onigine.
Circle intersect axes at say A(1,0),B(0,1),C(-1,0), and D(0,-1).
Let move from B along circular arc in clockwise direction when reached at A ,angular position change by 90 degree because coordinate axes are mutually perpendicular.If Y axes we take initial line then X axis is perpendicular to y- axis. Thus angular displacement is 90 degree.