what is meaning of this sentence?the measure of the exterior angle of a triangle is equal to sum of its two interior opposite angle? 

In ΔABC

∠A + ∠B + ∠C = 180⁰ [Angle sum property ] .....(1)

Also

∠C + ∠BCD = 180⁰  [Linear Pair ] ....... (2)

Equating (1) and (2)

∠C + ∠BCD = ∠A + ∠B + ∠C

∠BCD = ∠A + ∠B

Hence, exterior angle of a triangle is equal to sum of two interior opposite angles.