Our mind isn't a mere receiver of the thoughts , which gathers them all without any practical usage or without any calculation of its own, but rather works on a complex process it works out each and every information or thought it gets, calculates and reaches a conclusion. 
And when it comes to the fact of mind not being a vessel to be filled, but a fire to be kindled, undoubtedly it's cent per cent true. 
The statement itself has numerous meaning however all point to the fire, the ardent thirst , the quest for the ultimate goal of life - "Knowledge". 
Our mind isn't to be filled with knowledge- with facts and other's idea but we have to find knowledge on our own. 
In words of a philosopher - 'True knowledge isn't gained but made." 
Our mind is not a vessel that we copy someone knowledgable to get some of their ideas, their thinking and views . We need to realize that we ought to ignite our own mentality, our own intellect. 

The mind isn't a bottle to be filled with ideas gathered from the exterior world, but a piece of wood that has potentials of its own. It only needs a bit of kindling to set it in flames, to set it in a quest for truth and knowledge. 

Many great thinkers of the bygone era have had similar quotes. Socrates , William Butler Yeats and Plutarch included among them. 

When we give a little more thought to the saying, its amazing how very deep a meaning it offers. 
Consider this single instance- suppose Galilio didn't kindle his curiosity, suppose he filled his head with the wrong ideas. I don't need to tell you any further. 

So my friends as bearers of young blood, instead of cramming our heads with other's ideas , lets set our mind on fires, lets be original each one of us setting on quests of wisdom.

Hope this helps:)
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