We explain it as follows:
1. When θ is acute (<90), cosθ is positive. Hence work done is positive.
example: when a body falls freely under the action of gravity.
cos0 = 1. therefore work done by the gravity on the body falling freely is +ve.
2. When θ is acute (>90), cosθ is -ve. Hence work done is -ve.
example: when a body is thrown up, its motion is opposed by the gravity. the angle θ b/w gravitational force and displacement is 180. cos 180 = -1, therefore work done by the gravity on a body moving upward is -ve.
when, θ = 90
cos90 = 0
work done is said to be zero.
example: when a body tied to one end of a string is rotating in a circle, work done by the centripetal force applied along the string is zero because
θ = 90
Hope this will help you to understand the concept.