This question relates to one of the key central themes of the novel, which is the educational philosophy of Thomas Gradgrind and how he hopes to educate his charges as well as his children using an extension of his Utilitarianism philosophy, that only focuses on facts. This philosophy is summarised effectively at the very beginning of the novel:
"Now, what I want is, Facts. Teach these boys and girls nothing but Facts. Facts alone are wanted in life. Plant nothing else, and root out everything else..."
Chapter 2, then, in which your quote occurs, humorously refers to Mr. Gradgrind readying himself to pour forth his facts and describes him as a "cannon loaded to the muzzle with facts" and ready to "blow them [his students] clean out of the regions of childhood at one discharge." Dickens uses a metaphor to describe his students as "little pitchers" who are "waiting to be filled full of facts." Note how this approach to children imagines that they are "empty" and needing to be "filled." It does not identify or accept that children have their own way and approach to life and rather treats them as empty vessels in need of being filled.
by little pitchers are you refering to the 'little vessels' in the first chapter? If so, the little vessels represent the children who attend Gradgrind's faculty of fact. The reason why they are symbolised as such is because Gradgrind is basically filling them up 'to the brim' with his endless supply of facts and statistics. This aims to depict Gradgrind as a didactic and strict teacher who intends to wipe out any hint of fancy with the 'imperial gallons' of facts.
In Hard Times, Chapter one, we are introduced to the Utilitarianism educational system. Here, Thomas Gradgrind refers to his students as the "little pitchers" waiting to be filled with facts. To him, they were nothing more than empty vessels, which he was going to fill to the brim with fact, facts and only facts.