This expression is based on the expression for a circle , P = (cos t) i + (sin t) j. The 3 in the i term stretches the circle into an ellipse. Of course, the time t is measured in radians, not degrees.
Following is the graph of the motion of the terminal point of the ellipse.
The terminal point starts at t = 0 s at the position (3,0) and proceeds in an anti-clockwise direction. Its position at various times is indicated on the graph.