I think wikipedia explains it all:
A split infinitive is an English-language grammatical construction in which a word or phrase, usually an adverb or adverbial phrase, comes between the marker to and the bare infinitive (uninflected) form of a verb.
A split infinitive occurs in the opening sequence of the Star Trek television series: to boldly go where no man has gone before. Here, the adverb "boldly" splits the full infinitive "to go". More rarely, the term compound split infinitive is used to describe situations in which the infinitive is split by more than one word: The population is expected to more than double in the next ten years.
As the split infinitive became more common in the 19th century, some grammatical authorities sought to introduce a prescriptive rule against it. The construction is still the subject of disagreement among native English speakers as to whether it is grammatically correct or good style: "No other grammatical issue has so divided English speakers since the split infinitive was declared to be a solecism in the 19c [19th century]: raise the subject of English usage in any conversation today and it is sure to be mentioned". However, most modern English usage guides have dropped the objection to the split infinitive.