What is Rcos theta and Rsin Theta?

They are the projections, onto the x and y [Cartesian] axes, of a point whose polar coordinates are (R, theta).

It's a common Trig way to express a point when a radius is rotated around a given angle. For example, where exactly would the edge of a two foot gate lie if the gate opened 30 degrees? R is two feet. Two times cosine 30 is the x coordinate and two times sine 30 is the y coordinate.