It is hard to say because this is a very non-standard way of writing rules. My guess is that they mean "voiceless consonant" when they say "Cvoiceless"; and "Cɪ" means "C ɪ" where "C" is an abbreviation for "Any consonant" and "ɪ" is a high front lax vowel – thus "when followed by a consonant which is followed by a high front lax vowel".
"#__CC" is only meaningful as part of a rule, and literally means "when it is at the beginning of a word, and is followed by two consonants" (which, as it happens, are a "consonant cluster"). It depends what the "X→Y" part of the rule says, for example "i→ɪ" as the rest of the rule would mean "i becomes lax if it is at the beginning of a word and is followed by two consonants". But if the change part of the rule says "V→Ø" then that would mean "delete a vowel at the beginning of a word when....", and "Ø→i" would mean "insert i at the beginning of the word, when...".