Faber is explaining to Montag how books are very important in our lives, and have real meaning and quality. He give three reasons as to why books are important: They have quality, they allow the leisure to process their meanings, and the prompt people to act on what they've learned. When Faber brings up the Hercules and Antaeus legend, he is referring to how books have quality. Unlike a lot of other entertainment that shows only the beautiful, fantastical, entertaining, pleasing parts of life, books describe everything. They "show the pores on the face of life." They describe the good AND the bad, and are truthful, unlike many other forms of entertainment. And, because books are so grounded, so rooted in reality, they keep us strong. If we can learn about the good and the bad in life, we develop strength to cope with trials. If all we do is focus on the good, and refuse to admit that there are bad things, we are ill-equipped to deal with reality (take Mildred, for example, who tries to kill herself instead of dealing with her issues).
In the legend that Faber describes, Antaeus is a very powerful wrestler because he stood firmly on the earth. This is like everyone who reads books--they keep us strong, because they show us what life really is, and how to cope with life realistically. But as soon as Hercules lifted Antaeus off of the earth, he perished. This is akin to us refusing to read books; we are not grounded anymore, we have no sense of what life is really about, and we can't cope with bad things. This will destroy us; it makes us weak. We need to, like Antaeus, stay firmly rooted in reality to make us strong, and books help us to do that.
It's a rather deep connection that Bradbury is trying to make, but I hope that I explained it well enough to help you understand. Good luck!