In Part Two of the novel, Montag rides on a subway train to visit Professor Faber. Montag is seeking Faber's advice and help on how to comprehend information in the texts that he has been reading. While he is on the train, he attempts to read a passage out of the Bible but is continually interrupted by a loud Denham's Dentrifice advertisement blasting through the speakers.

Montag's inability to grasp and comprehend the information that he is reading relates to one of his early childhood memories. As a child, Montag visited the beach with his cruel cousin, who bet him a dime that he could not fill a sieve with sand. Montag proceeded to quickly dump more and more hot sand into the sieve, only to watch it fall through the bottom.

Montag's inability to fill the sieve with sand metaphorically represents his inability to comprehend and remember the information he is reading. In this metaphor, the hot sand represents the information in the texts. Similar to his experience filling the sieve with sand, Montag's attempts to increase his reading speed are in vain because he cannot grasp or comprehend the specific information he is reading.

The episode of the sieve and the sand for Montag in Fahrenheit 451 is comparable to his inability to understand what he reads. 

Just as he couldn't fill the sieve no matter how fast he filled it with sand, Montag, no matter how much or how fast he reads, can't comprehend it, at least not at the time the narrator tells about the incident. 

But it is also indicative of what Montag must try to do--read as fast and furiously as he can so that he can experience the reading before it is taken away.  If he reads fast enough, maybe he'll fill the sieve, figuratively. 

Montag is already trying to memorize what he reads while on his way to Faber, during the scene in which the memory of the sieve and the sand is revealed, but he keeps getting interrupted by a commercial.