The second section of "Fahrenheit 451", entitled "The Sieve and the Sand" refers specifically and literally to a passage in that section where Montag has a memory from his childhood. He remembers "trying to fill a sieve with sand," because a cousin had said that he'd give Montag a dime if he could do it. But, "the faster he poured, the faster it sifted through," leaving it completely empty. So, that is the literal meaning.
Symbolically, the sieve represents Montag's desire to pour books and knowledge into his brain, but how "he read and the words fell through" without him remembering any of it. He just kept pouring and pouring, but none of the words, or the wisdom, stuck. At this point in the novel he was on the subway trying to read a passage from the bible, but the noisy commercials on the subway, and all of the busy distractions of his society made it nearly impossible to concentrate, so he was having a hard time. So he was skimming words super fast and trying to absorb them before he had to give the book back to Beatty the next day, but it wasn't working. His mind was like the sieve, the words like the sand.
Another possible symbol for the sieve and the sand is how in Montag's society, they pour all sorts of information at the kids, but it is useless information that doesn't matter, that isn't relevant to life or happiness. They do this in order to make people "feel they're thinking...and they'll be happy." Their society pours and pours concrete factoids so that people think they are getting knowledge, wisdom, information and intelligence, when really, it just sifts through and leaves them empty, because it is all useless information (like sand, instead of weighty and substantial items).
Those are a couple thoughts-I hope that they help! Good luck!