A 2 year bond, yield 6%. A 1 year bond, yield 4%. What's the implied rate for the bond that starts one year from now?
(1+0.06)^2 = (1+0.04)*(1+x) Solve for x
The left hand side represents the gross return on a 2 year 6% bond
The right hand side represents two gross returns in sequence: first we hold a 1 year bond, then (for the next year) we hold a 1 year bond (that starts a year from now) with unknown return x
By expected interest parity (or by "the Expectations Hypothesis") the two sides have to be equal, there is no advantage (or disadvantage) to holding the 2 year bond or the two 1-year bonds in sequence.