WHY THE POET WANT US TO DREAM AND YET NOT MAKE DREAMS OUR MASTER?
First of all this query should be in the Literature category (main); and am sure if you've read the text of the poem''If-'' , by Rudyard Kipling, carefully ,then you should have no problem in understanding the difference that the poet/writer highlights in this expression, that you cite.
A.. Dreams are important, special things, Kipling suggests, they make many things happen, they inspire and lead and guide people and their achievements and goals in life. Without such dreams to lead us on and provide us directions, we are nothing and our lives are without end or purpose. Thus, within these parameters, it is good and positive to dream.
B.. On the other hand, however, dreams should not be allowed to overpower us, to enslave us and drive us into any sort of obssesive state or condition, which is negatve- indeed, if we let dreams control us (i.e rather than as in point A , controlling/utilising them) then we are more likely to lose directions, and to give up action and just sit in a reverie imagining things without real, material gain at all. This is the category of 'lost' dreamers, the poem/poet seem to suggest.
I hope you are able to differentiate between mastering and controlling our dreams and being mastered/controlled by them, now? For further detaild analysis and criticism, of the poem and its content, please see the 'Key Questions' on the right hand side here, in the yellow box; and also the Popular Links and Recoomended Questions in the Blue Boxes underneath.
I also hope the official Kiping Society link and the other ones are useful too, good luck.