Complex sentence: The probability that an infinite randomly generated string of text will contain a particular finite substring is 1, but this does not mean the substring's absence is "impossible", despite such an event having a probability of 0.
Split into simpler sentences: The probability that an infinite randomly generated string of text will contain a particular finite substring is 1. However, this does not mean the substring's absence is "impossible", despite the absence having a prior probability of 0.