A split infinitive is an infitive with another word, usually an adverb, inserted. Thus 'to boldy go' is a split infinitive from the verb 'to go'.
There is a myth circulating among uninformed English teachers that the split infinitive is somehow incorrect, yet if you ask them they cannot come up with a reason besides "it's wrong". The truth of the matter is that infinitives have been split for nearly seven hundred years, and condemnation of this construction arose in the 1800s by applying Latin to English grammar -- a practice which in the age of modern linguistics is laughable. Language does not conform to arbitrary rules, much less rules from another language.
The next time your English teacher tells you the split infinitive is wrong, ask them why. I promise you will find the non-response hilarious.