जल means water and मन्थन means churning. This Nyaya' is conceptualise on the basis of the activity of churning water.
We get butter, churning curd. Considering this we get an idea/ a lesson that the activity of churning yields some fruit. But knowing this 'Nyaya' if someone tries to churn to water, he cannot get anything. In other words, (churning of curd may be beneficial, but) churning of water is always useless. This 'Nyaya' is conseptualised to give this advice. A person must understand very well that he must not spare time or use his ability to do the thing which will not give any concrete result/fruit.
For example, a person when falls sick must go for medical treatment. (Here we have to think of churning as medical treatment, a string and curd as medicine). In this activity the medicine that helps to cure disease, if consumed can yield us the fruit in the form of curing the disease.