Infinitives are normally made up of two words--the word to followed by a verb in the present tense. Some stylists say that to "split" the infinitives, that is, to place a modifier between the to and the verb is bad grammar. But sometimes it is the perfect place to put an adverb.
Most European languages, including Latin, have single-word infinitives. Many grammatical terms and rules do come from Latin. Some people thought that since it was impossible to divide a Latin infinitive, that we should avoid it in English even though our infinitives consist of two words instead of one.
A writer can confuse the reader if the to and the verb are separated by a long phrase or clause. This is true. But in English it is appropriate to thoughtfully place an adverb between the two parts of the infinitive.
Some editorial guidelines, especially in England, still insist on this "rule."