An expression of Biblical origin.
As mentioned above, comes from the Biblical injunction not to "cast your pearls before swine." Pearls, of course, are considered to be things of great value, and swine (pigs) are often considered to be lowly animals.
Thus, the phrase translates as "Don't waste something valuable by giving it to someone who doesn't/can't appreciate it." This can apply to words/thoughts/wisdom as well as physical objects, and the expression is frequently used in a condescending fashion.
As for the comic, the title might ironically mean that Pig actually possesses the higher moral qualities that Rat is incapable of appreciating -- or even more ironically, that the things the artist is trying to communicate in the strip will be lost on the general public reading it.
"I read that beautiful Shakespearean sonnet to her, but it was pearls before swine to such an illiterate person."