Is working out light weights bad?

recently i have been getting pretty serious about lifting. i work out three times a week at home but all i have is a chin up bar and adjustable dumb bells that go from 5 lb to 15 lb. i have been needing to do more and more reps in order to really feel the workout. i have heard that doing many reps of small weights isnt as effective as doing few reps of heavy weights. is this true?