What did Bacon mean in the line "Some books are to be tasted, others to be swallowed, and some few to be chewed and digested?"
Bacon opens the essay, "Of Studies," with this line:
Studies serve for delight, for ornament, and for ability.
This notion parallels the line quoted in your question. Those studies that delight are meant for private reading and personal contemplation, amusement, and insight. Those studies that are for ornament are used to improve discourse, speech, and writing. Those used for ability are used for the "judgment, and disputation of business." This is a simplistic interpretation of these first lines because this is the correct interpretation in the context of this essay. However, outside of this essay, I assume Bacon would agree that one can read for amusement or leisure but also reach a profound insight that could be used in serious learning.
So, those studies that delight would tend to be "books that are to be tasted." You don't really take in profound information; you simply taste it. There are no profound philosophical ideas that would require you to imbibe or swallow these ideas. Those studies that are swallowed are a bit deeper. They have more substance. They contain more wisdom than something light and amusing which would be used simply for tasting. The books that are to be chewed and digested have real philosophical, sociological, or psychological insights that become a part of your mental being. When you chew and digest something, you absorb the nutrients. Those nutrients become a part of you. Bacon is saying that only the most significant, truthful, useful, and worthy books should be chewed and digested. If they are superficial, practically useless, or lacking in truth, they should only be tasted.
In his essay "Of Studies," Francis Bacon makes a distinction among books. When he writes that "Some books are to be tasted, others to be swallowed, and some few to be chewed and digested," he distinguishes among the quality and value of the content of books.
Bacon states that the use of a book should be in equal measure to its value. That is, some books are only worthy of a quick reading or an examination for a particular idea (they should only be "tasted"). However, those books that are important must be read thoroughly (they should be "swallowed"). Others are profound in their thought and meaning; these need to be read slowly and thoughtfully (they should be "digested") in order for the reader to be able to truly comprehend and understand the nuances of meaning, as well as the implications of these defining points. These are the books that the reader will return to so that he can consume even more of their content. The ideas contained within such books can become a part of his mind. In this way, the reader can raise his level of thought and enrich his thought processes so that his new ideas will be of great merit. Also, "reading maketh a full man." Reading books of great value truly enriches a man just as good food makes him healthier.